One of Tom Lubbock's nicest habits as an art critic was to observe some quirk in a piece of art and, while distracting you with this eccentric detail, to whip off the magician's cloth and illuminate the whole work. It was a regular trick of the short essays he wrote in his "Great Works" series for the Independent: a preparatory manoeuvre priming you for the big picture.

Published posthumously as a collection, Great Works deepens a neat journalistic technique into a profound way of seeing. Why did Alfred Hitchcock, in his 1941 film, Suspicion, put a light inside the glass of milk in Cary Grant's hand? And what does this little special effect tell you about Francisco de Zurbarán's Still Life with Jars, painted in 1635? Lubbock's unlikely juxtaposition lights up not only a film and a painting, but the very idea of the illusion of light in art.

Or he wonders which patch of yellow it was that Proust noticed in Vermeer's View of Delft. In À la recherche du temps perdu, this is the last detail the novelist Bergotte thinks about as he dies in an art gallery. Lubbock notices that Proust must have slightly misremembered the yellow patch when he put it into Bergotte's mind. The intensity of the detail  of any detail  is an illusion created by its place in the composition. Bergotte dies clinging to an illusion as the wider composition fades. This is why it is such a good image, Lubbock writes, of a man's "fading consciousness and will to live".

As Laura Cumming points out in her introduction to Great Works, Lubbock wrote the essay while waiting to undergo surgery for the brain tumour that would kill him. In September 2008 he was diagnosed with an advanced glioblastoma multiforme, situated in an area of the brain involved with the production of language. For two years he wrote, knowing that he would, soon enough, lose the ability to speak and to write. And he wrote of his own fading in the same way he wrote about art  fascinated by the quirks in himself, charming his readers into seeing the big picture.

The quirk of the brain as a biological entity is that it can register pain from everywhere else in the body but not from within itself. Although Lubbock experienced fits, he was unaware of them as they happened. His tumour was painless, so he could observe in himself the oddness of dying without physical suffering: "It's wholly a matter of knowledge and how I deal with this knowledge." The time he had to contemplate his death was a foreshortened version of the knowledge we all have.

Until Further Notice, I Am Alive  composed from his journals and a long article written for this newspaper  is an account of what William Empson called "the human practice of dying". And nothing in Lubbock's writing indicates that he did it with any less of the humour and intellectual curiosity that he lived by. He finds it slightly odd that, even though he has a small son who will grow up without him, he doesn't aggrandise his impending paternal absence. "I'm not made for gloom," he writes. "Not cast for catastrophe."

His wife, the artist Marion Coutts, reinforces this honourable and moving trait of equanimity in her introduction. "What we needed to know here," she writes of their post-diagnosis life, "was an extension of what we knew already."

Of course, there is sorrow and loss and terror. Instead of envying the young their youth, Lubbock finds himself envying the old for all the additional years they have enjoyed. Yet he can also find moments in which he feels a sense of blessing in the change in life forced on him by his tumour. His thoughts turn to his wife, and he asks if he has ever written her a proper love letter. He writes one. "I send you all my love from the middle of the night. Hold on to me. Hold on to us."

The story of the book is simple and clear: first operation, diagnosis, treatment, good news, bad news, second operation, then more bad news. Lubbock is not preoccupied by the science of his condition, nor tempted by far-fetched remedies; rather he is interested in the effects produced by this most abstract of afflictions. He is amusingly wry about the attitudes of others. When confronted by those who can't understand why he isn't investigating every alternative treatment, he detects a hint of disapproval and gently puts it down: "I'm not going to count my death as a personal failure."

Above all, what he notices is language as it slips away from him or reveals itself to be bizarrely involuntary, as if some "inefficient proxy" were forming the words. When the tumour distorts his speech, he recognises that, even in the midst of his jabberwocky, "the stress of the words and sentences  sense or nonsense  is equally and perfectly accurate." It is an observation of the linguistic structures underpinning mental existence. "Loss of speech amounts to the loss of mind," he writes. "Mind means talking to oneself."

Yet, later on, having recovered from a period in which words utterly failed him, he notices that "my experience of the world is not made less by lack of language, but is essentially unchanged."

In the silence at the end of his book, Lubbock leaves an eerie trace of that ineffable thing we all leave behind, summoning up the big picture in his two final words: "the world".

The quirk I notice in this review is that I have referred to events leading up to Tom Lubbock's death in the past tense, but when I cite what he wrote, I instinctively revert to the present, as if he were still writing. Until Further Notice, I Am Alive is an illusion of the author as a living composition. The details are beautifully observed; the big picture is illuminated; the title is well chosen.

The art critic Tom Lubbock died in January 2011, aged 53. A tumour shut down his brain. The first sign that this would happen had come 30 months before, when a fit overtook him as he, his wife and infant son were visiting friends outside London. Until Further Notice, I Am Alive is an occasional journal that Lubbock shaped into book form during that time, aware that his diagnosis was terminal. As you read your way through and approach the back cover, life and the book you're holding close down together. Thinner and thinner grow the sheets between your fingers, the print giving way to blank  and you're up against the hard binding. Lubbock's working experience gave him another analogy for the situation. "Mortal. We occupy a limited patch of space (I have never believed in travel) for a limited stretch of time. Like the art of realistic painting: pictures hold an equivalent in the confined areas which they enframe."

As the tumour gradually wore away at his hold on speech, Lubbock went on with his job, delivering weekly essays to the Independent about those enframed areas that painters create. Great Works (Frances Lincoln, £18.99) selects 50 pieces from a series the paper ran for five years, the last of them composed three months before his death, while he was noting in his journal: "My language works in ever-decreasing circles  It's very difficult for me to talk at all." It is an endlessly lively and surprising book. The format's simple: reproduce a picture, usually by a well-known artist; talk about it for two or three pages; add a nifty 100-word thumbnail about the artist. But the title misleads. Lubbock writes not to celebrate "greatness", but to plunge head first into strangeness.

Again and again Lubbock comes at an artist from an unfamiliar end of his oeuvre. Not a screaming pope by Bacon but a sand dune, Giotto's allegories of vices rather than his gospel stories, not a shouty melodrama by Delacroix but a Still Life with Lobsters. "Lobster, lobster, lobster, lobster, lobster," Lubbock's commentary begins, insistently nonsensically. He wants you to register the sound of the word, becoming "stranded and estranged" from its meaning. This, he suggests, is an effect that the still life delivers visually when it heaps lobsters with dead rabbits and pheasants on a country hillside. No narrative sense can be made of the scene. Yet from that absurdity, Lubbock leaps to a powerful inference: that for Delacroix, "a picture is a set of functions, a series of boxes to be filled. The world is a supply of things and scenes with which to fill them, quite arbitrarily." That bites: the painter's grandes machines such as Liberty Leading the People start to look different in the light of those lobsters.

But the insight also shines back on Lubbock's own way with painting. Insofar as he has any characteristic approach (in fact, Great Works is a virtuoso display of variety in essay technique), it is to treat pictures as spaces that "enframe" certain things and not others. Why did the painter put this in, not that? "You can never just subtract a part from a painting. You have to fill it in," he writes, wondering how Van Gogh's Wheatfield with Lark would look without the lark. His "scissors and crayon" stand poised: alongside his journalism, Lubbock was also a superb collage artist. Working out why the lark or the lobster happens to be there, he takes you closer to the painter's mind, and in this task he is very sure-footed  you invariably learn a lot. He enjoys steering your sensibility, too: listen to him sequencing epithets for the heroic-worker paintings of Léger: "Safe, graceful, optimistic, but tough, and never simply jolly."

Yet individuals and their achievements are not the main objects in view. What the collection keeps returning to is the way that "painting remakes its world from scratch". The stuff of painting is not like the stuff of that other 3D world, even when painting pretends to be realistic. "In the pictured world, the laws of nature change" and things are no longer held down by everyday gravity, optics or narrative coherence. What's more, mainstream western painting, according to Lubbock, sidelines great swathes of experience  relationships, children, sex, messiness and laughter. It still has content, certainly. From its own peculiar angle, it reflects with continual wonder on the fact that we have  or that we are  bodies. But it does so by conducting a set of visual thought-experiments. The way that Lubbock describes those experiments may not dwell on the turps and the sweat of the studio, but it communicates something rarer to come by, the liberating clarity of a philosophic mind.

His terse phrasing is heady. Of Hopper's Early Sunday Morning: "The scene says 'etc'." Of Zurbarán's still lives: "Everything feels seen." Behind those compressions lie vast resources of cultural knowledge, capriciously paraded  from Mickey Mouse to Dante, from Simone Weil to condom packets. What culture a writer carries matters also, in a sense, when he describes how he goes about dying.

Since we're all headed for the same door, the interest of the exercise turns partly on the different directions we're coming from. Until Further Notice, I Am Alive concerns the mortality of a highly educated Londoner, whose circle includes fellow journalists and some of the medics busy tackling a tumour that is immediately identified as malign. When Lubbock sleeps in a hospital ward, the night noises remind him of a Bartók concerto: he mulls over his Brecht and William Empson; he makes savvy, sceptical asides about other contemporary death writing, such as that of Julian Barnes and Joan Didion. Death approaches him at a point when he is, on his own terms, at the top of his game. He agrees with an art critic friend, Laura Cumming, when she remarks how "clearly defined" his life has appeared. All we see of his family is his little son and his wife Marion Coutts  who contributes a moving introduction-cum-afterword. Apart from them, no other ties come into view.

The tamped-down controls enable Lubbock to concentrate on his real-time thought-experiment: how to live staring non-life in the eye. He wants not to be evasive. Some friend sends him a book of remedies involving meditation, but "being asked to change my life and my self, in order to save my life" is something he rejects. "I side with 'western' medicine because it treats me as an organism." He wants to exist as a measurable body: that way, the tumour cannot constitute any kind of reproach.

He has an ear for religion, much as he has an ear for poetry, but God is not real to him  and, more important, God is not quite relevant to his present demands. God cannot confirm for Lubbock that "here" is wholly "here". And that is what he is intent to hold on to. "The prosaic, material, solid, opaque, secular, untranscendent, this-worldly": it is to these qualities he's "attracted", he writes. He loves this world, he finds it good, and adds: "I'm not made for gloom." And yet to be "attracted" to something, you must be somehow distinct from it. Knowing he'll die, he becomes acutely aware of seeing the world from a certain angle, and: "I can't imagine the world's good as apart from my perspective."

Equally, how can his powerful mind grapple with its own status as a physical brain, under attack in its left temporal lobe? Philosophy's big riddles get dramatised here in rich detail. Lubbock does not dwell on physical pain but he tries to track, with awesome stubbornness and lucidity, the gradual disintegration of his own speech patterns. He heads into strange territory. Six months before his end, he is contemplating "the mystery of summoning up words. Where are they in the mind, in the brain? They appear to be an agency from nowhere. They exist somewhere in our ground, or in our air. They come from unknown darkness."

Early on in his to-and-fro with operations and chemotherapy, Lubbock wrote touching love letters to his wife and a son he feared might be too young to remember him. But tearjerking was not really his business. What gradually came on me as I read his notes and meditations was a sense that behind the witty and complicated man I very slightly knew, there stood a kind of hero of contemplation. Always Lubbock's instinct is steadily to wonder. One page before the print surrenders to blank paper  exiting on a keel of high poetry  he remarks that language is slipping away fast, and yet that his thoughts when he looks at the world are "vast" and "limitless". And characteristically he adds: "This is curious."

 Julian Bell's Mirror of the World: A New History of Art is published by Thames & Hudson.