A colleague raised her eyebrows when I suggested that I review this slim volume, number 76 in the latest series of Penguin's Great Ideas. "They are," she said, "only bits of books, aren't they?" She does have a point. Few of them are complete in themselves: most are plucked, without any apparatus, from larger works. The absence of annotation or introduction, a policy decision designed to minimise the distance between text and reader (and not at all, heaven forfend, for reasons of cost), can have the unintended effect of making readers feel as if they have been thrown in at the deep end.

But one can make an exception in this case. For you can get away with calling a selection of essays a complete book, even if it's barely more than 100 pages long; it has a very witty design (the motif on the front cover only a quarter filled in); and it is, after all, by RL Stevenson.

Time was when just about every civilized bookshelf in the land could be counted on to house a copy of his Virginibus Puerisque, the collection which contains the title essay of this little book (and two others). When I discovered this a few years ago, I marvelled at how pleasant it still was to read, how wise, how gently engrossing; was this, perhaps, why they had fallen out of favour? Two of the other authors in the latest Great Ideas are Schopenhauer and de Maistre; Stevenson is as far away from them in tone as it is possible to get. Stevenson is the great "aye sayer", capable of finding pleasure in the smallest things, even when he is contemplating "the Enjoyment of Unpleasant Places" - "I am happier where it is tame and fertile, and not readily pleased without trees," he says, but "wherever a man is, he will find something to please and pacify him ... Let him only look for it in the right spirit, and he will surely find." One imagines that Stevenson could find something uplifting on a wet Sunday afternoon in Merthyr Tydfil, widely considered the most challenging of loci in the British Isles for the optimist.

Reading Stevenson's essays is nothing so much like having a stroll in the company of an amiable and witty companion, the kind you want to do all the talking, not because he's so much cleverer than you, but because he notices the little things you might not have thought worth describing yourself. He is on the side of the dreamer, the dawdler, and, as the title suggests, the idler. He contemplates, and cheekily revels in, the resentment of the industrious for the "cool persons in the meadows by the wayside, lying with a handkerchief over their ears and a glass at their elbow. Alexander is touched in a very delicate place by the disregard of Diogenes." You don't need footnotes to get the gist of what he's saying there.

You can trust him, too, on the larger subjects. His "On Falling in Love" may be barely 12 pages long, and no one's ever going to have the last word on the subject, but has it ever been more charmingly described than this? "Love should run out to meet love with open arms. Indeed, the ideal story is that of two people who go into love step for step, with a fluttered consciousness, like a pair of children venturing together into a dark room."

He was, if I have got my facts right, only 30 when that essay was written (and, tragically, 44 when he died), but there is something ageless about his observations. His "Crabbed Age and Youth" is as sympathetic to the effect of time on us as it is to the playfulness and spirit of childhood. You will learn, in it, the secret of how "to be a good artist in life and deserve well of yourself and your neighbour."

"Books are good enough in their own way, but they are a mighty bloodless substitute for life," he says; but if you find yourself in one of those inescapable life-denying environments - a stationary queue at the Post Office, a windswept train platform seemingly abandoned by your train - then all you have to do is pull out this perfectly-sized booklet and be transported into a world of sunny good sense.

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This is a book about "the most important process on the planet": photosynthesis. Plants grow by "eating the sun", trapping its energy and using hydrogen from water and carbon from air to produce flowers, fruit and seeds. The "scrap of sunlight" converted into organic matter by the world's plants each day is equivalent to the energy in the global arsenal of nuclear weapons. But, by releasing the energy locked away some 300m years ago in fossil fuels, we have upset the delicate balance of the carbon cycle and made "the atmosphere itself as artificial as a Capability Brown landscape". From molecules to the planetary scale, Morton's beautifully written book reveals how life is made from light. The living landscapes we inhabit are shaped by photosynthesis, and Morton's sense of wonder at the pervasive influence of this process is nowhere stronger than while walking across the South Downs near his home: "It's grassland like this, more than any other habitat, that gives us both homes and horizons." A rich and wide-ranging study.