Flaubert once wrote to Turgenev: "Never have things of the spirit counted for so little. Never has hatred for everything great been so manifest  disdain for beauty, execration of literature. I have always tried to live in an ivory tower, but a tide of shit is beating at its walls, threatening to undermine it." It's a favourite quotation of mine for many reasons  among them the way it is both so pompous and so not-at-all pompous at the same time  and of course every serious writer thinks exactly the same of his own age. But all the same, so weary, stale, flat and unprofitable seems the mood of the publishing industry at the moment, and so swollen and celebrated seems the appetite for the ill-bound crapola of the departure lounges, that it is tempting  after reading something as fine as The Finkler Question  not to bother reviewing it in any meaningful sense but simply to urge you to put down this paper and go and buy as many copies as you can carry.

But let's press on for now. The Finkler Question (longlisted for this year's Man Booker prize) is full of wit, warmth, intelligence, human feeling and understanding. It is also beautifully written with that sophisticated and near invisible skill of the authentic writer. Technically the characterisation is impeccable, the prose a subtle delight, the word selection everywhere perfect, the phrase-making fresh and arresting without self-consciousness. Indeed, there's so much that is first rate in the manner of Jacobson's delivery that I could write all day on his deployment of language without once mentioning what the book is about. A single line describing the hero's father will have to do: "a man who stood so straight that he created a kind of architectural silence around himself".

So what is the book about? Well, this is the story of Julian Treslove, once of the BBC (pleasingly satirised) and now making a living as a celebrity lookalike. Treslove is not Jewish but, in simple terms, the narrative details his love affair with and besotted inquiry into what Jewishness means  politically, socially, economically, romantically, intellectually, emotionally, culturally, musically and so on. Treslove has only a "timid" awareness of his place in the universe "ringed by a barbed wire fence of rights and limits". He wants to be part of something vast and ancient, something abounding and intense. He wants to be Jewish.

This is also the story of Treslove's longtime best friend and worst enemy, Sam Finkler, a spurious (therefore popular and successful) philosopher who is forever on television, Desert Island Discs and so on. Finkler is Jewish and Treslove uses the word "Finkler" to mean "Jew"  hence the title of the book comes to mean "The Jewish Question". Finkler is an Israel-hater and joins a group of celebrity Jews called Ashamed.

Then there's Libor Sevcik, an elderly ex-Hollywood journalist who is in mourning for his beautiful dead wife and a former teacher to Treslove and Finkler. His Jewishness is long marinated in Schubert impromptus and described as intellectually "ritzy". He's pro-Israel.

And finally, of the four main characters, there is my favourite, Tyler. She is the wife of Finkler and turns out not to be Jewish despite having so studiously learned the practices of the faith and so completely taken on the demeanour of a certain sort of Jewish wife that when Treslove sleeps with her he is staggered and disappointed to discover her secret. She also dies.

So then: all this Jewishness and not Jewishness is what the book asks the reader to weigh  eloquently, farcically, seriously, mockingly, perceptively, dumbly, brilliantly. Now, here's the rub. (Finkler and Treslove are obsessed by Hamlet by the way.) I am not Jewish. But the reason I enjoyed this novel so, aside from its formal verve, is because it seemed to me that Jacobson is only using Judaism as his "way in". In other words, as all serious artists do, he is mining his immediate milieu as a way of directly unearthing the deeper questions of family, society, belief, culture, relationships  the underlying nature of humanity.

Of course, on the part of non-Jews, there is a reluctance to say what they think about Jewish concerns and what Tyler refers to as their "self-preoccupation". ("Finklers" are described as notoriously "touchy".) But I've been reading a lot of anthropology recently and you only need a couple of pages on the amazingly close-relatedness of the entire human population (one original family of some 200 out of Africa) and a couple more on the way Homo sapiens is compelled to make up rituals by way of fashioning meaning where there is none to realise that on one level all the great religions  yes, Judaism too  are complete bullshit.

Except, of course, that they are also the most powerful, important and consoling mythopoetic way we have of understanding our lives, our circumstances, our random births and our brutal deaths. And that is what the book is really about: the pompousness and not-at-all pompousness of religion; or, to put it another way, the overwhelming bullshit and the overwhelming beauty of Jewishness, where Jewishness is a metaphor for human culture in general.

Edward Docx's most recent novel is Self Help (Picador).

Julian Treslove is a man whose sense of identity is so fragile that he has fetched up earning a living as a lookalike  not because he resembles anyone in particular, but because he looks like all sorts of people in general. He doesn't greatly care for it, but then he didn't greatly care for his career at the hated BBC, which foundered because of his propensity for playing too much mournful music on Radio 3, nor for his subsequent makeshift jobs as a removal man, shoe salesman or sash-window replacer. He has, on the other hand, cared for a lot of women, but not in a way that induces any of them to stay with him for very long. Half-terrorised and half-seduced by the fantasy of their untimely ends, he exists in "a perfected dream of tragic love" heavily influenced by Italian opera; wistful admiration for La Bohème has even led him to suppose, rather proudly, that he suffers from a Mimi Complex. "Don't I look after you when you're ill?" he asks one retreating lover. "You do," she replies. "You're marvellous to me when I'm ill. It's when I'm well that you're no use."

But perhaps the worst misfortune that can befall someone desperate to star in their very own tragedy is to be upstaged. Unluckily for Treslove, his two oldest friends  the showbizzy expatriate Czech octogenarian Libor and the self-regarding pop philosopher Finkler  have recently been widowed. Quite aside from being overwhelmed by grief on their behalf, he is also emotionally blindsided by the vanishing possibility of being similarly bereaved. "Greedy little grave robber," as the mother of one of the two sons he can barely tell apart remarks. "Why can't he allow other men to mourn their own wives? Why must he always get in on the act?" But if the usurpation of intimate grief is a knotty enough moral problem, it's about to be taken to an entirely different level.

Libor and Finkler are both Jewish; Treslove is not. In fact, he conceives of Finkler as so total a representation of Jewishness that he uses his name to stand in for the word Jew: "It took away the stigma, he thought. The minute you talked about the Finkler Question, say, or the Finklerish Conspiracy, you sucked out the toxins." An uncharacteristic delicacy prevents him from explaining this to Finkler, although he is unable to resist confiding in his friend when he believes  on highly dubious grounds  that he has been the victim of an antisemitic mugging, an attack that has left him both strangely cheerful and newly purposeful. Before long, he is trawling the internet for accounts of violence against Jewish people across the globe ("I'm not saying it makes pleasant listening," sighs Finkler, "but it's not exactly Kristallnacht, is it?"), joining Libor for a Seder dinner and hooking up with Hephzibah, a woman who is about to open a museum of Anglo-Jewish culture. In short, he is trying to become a Jew.

Jacobson has long made it his fictional business to set up and explore high-risk scenarios; to invite our offence and defuse it with a defiant "Ha!" in the face of our fastidiousness. After all, his last novel, The Act of Love, revolved around a man's determined attempts to provoke his wife to infidelity. He has also repeatedly returned to the subject of Jewish identity, turning over themes of religious and social persecution, group mentality and assimilation and exclusion with intense and often painful humour. In this new novel, recently longlisted for the Booker, he creates a protagonist so desperate to locate a sense of belonging and acceptance that he is driven to introject the history and culture of an entire people. And that, as Jacobson demonstrates, is easier said than done.

There is, of course, fantastic sport to be had with Julian Treslove, and Jacobson duly has it, from Julian's dull-witted attempts to anatomise Jewish humour  there is a wonderful episode in which he fails woefully to make Hephzibah laugh, despite throwing in "everything he had  a shrug, a 'so', a 'now' and an extra exclamation mark"  to his growing obsession with the effects of male circumcision on sexual pleasure. He is, for much of the time, a relatively benign stereotyper, a hapless tourist in a world that he knows will never really be his. And neither are we without sympathy for him: for his emotional stuntedness, his besetting blandness and his rather understandable desire to pre-empt catastrophe before it hunts him down.

But these are the lightnesses in a novel that has set itself the task of capturing the conflicts and contradictions that, in Jacobson's view, characterise the position of contemporary British Jews. While Treslove fiddles around with salt beef sandwiches and klezmer bands, Libor and Finkler find themselves, without the protection of marital and romantic love, cast into a world of such ambiguity and complication that their own identities, initially so solid-seeming, come under threat.

Despite the deliberately knockabout, throwaway humour of his prose, Jacobson sets out their stalls with great care. Libor preserves the idea of Israel  "Isrrrae" to him  as a place in which history might one day force him to seek refuge; Finkler will not even countenance the word, referring to it only as Palestine. For Finkler, the conduct of Israel is a moral aberration on which justice must be brought to bear; for Libor, it is more akin to the regrettable behaviour of an errant family member. Libor peppers his conversation with Jewishisms the better to irritate Finkler; Finkler retaliates with cod-philosophical analysis. When Finkler prefaces their lengthy arguments with an eye-rolling, "Here we go, Holocaust, Holocaust", Libor retorts with an equally resigned, "Here we go, here we go, more of the self-hating Jew stuff."

If Finkler will not admit to self-hatred, he will certainly admit to shame, and consequently joins an organisation called Ashamed Jews to protest against Israeli actions; it's a side benefit, as his wife scathingly points out before she dies, that it's littered with celebrities and does no harm to his public profile. But if Finkler's position is hardening, so, too, in a different way, is Libor's: when an old friend appeals to him to help her bring attention to the plight of her grandson, blinded in an antisemitic attack, he finds that he cannot. It is not, he explains, a matter of compassion fatigue: "You can't feel compassionate towards yourself or towards your own. It's more fiercely protective than that. When a Jew was attacked, I was attacked . . . But it's too long ago now. Too long ago for us, and too long ago for those who aren't us. There has to be a statute of limitation."

But what Libor means is that it's too long ago for a man reaching the end of his life, bereaved, failing, unable any longer to participate in a seemingly endless dispute over moral, spiritual and literal territory. It is not too long ago, runs the novel's counter-argument, for a blinded grandson, nor for Hephzibah, who arrives at her museum one day to find its door handles encased in rashers of bacon. Nor, indeed, for Finkler, stopped in his tracks by his own son's physical assault on a group of Zionists  "settler types", as his son calls them  protesting at a political debate on Israel's right to exist. But aside from the revelation that his son is "a fucking little antisemite", what affronts Finkler most is his inability to accept debate, to allow the continuance of the tradition of dispute and discussion that has characterised his relationship with Libor and, it is suggested, Jewish life throughout history.

The Finkler Question is a terrifying and ambitious novel, full of dangerous shallows and dark, deep water. It takes in the mysteries of male friendship, the relentlessness of grief and the lure of emotional parasitism. In its insistent interrogation of Jewishness  from the exploration of the relationship between the perpetrators of violence and hatred and their victims, to the idea of the individual at once in opposition to and in love with his or her culture  it is by turns breezily open and thought-provokingly opaque, and consistently wrong-foots the reader. For Treslove, the committed shape-shifter with little really at stake, such demands unsurprisingly prove rather too much. "Would he ever get to the bottom," he wonders, "of the things Finklers did and didn't do?"