It was James Dyson, inventor of not only a vacuum cleaner but also a hand dryer, who, when struggling to come up with the most abstruse and pointless subject one could study at university level, came up with "French lesbian poetry". How, then, he would love this book. Although, alas, it contains no separate entry for French lesbian poetry, it does have two closely-set pages on lesbian poetry (not to mention gay poetry and queer poetry), from which he might learn something, although I doubt he would be inclined to. Which would be a pity if my hunch is correct, for the very word "poetry" comes from the Greek "poesis", meaning "making" (cf the Scots term "makar"). We learn this from the entry "Poetry", which, although it could, in Borgesian fashion, itself be exactly as long as the book, confines itself to two and a half pages  as much, you will have noticed, as we get on lesbian poetry.

To acknowledge that a poem is a thing made and therefore crafted may come as a surprise to those children who have been encouraged to write poems whose sole requirement is that the lines do not extend to the right-hand edge of the page. Even that, though, involves a necessary constraint  and even that constraint need not apply if we consider prose poetry (whose generally-agreed starting point, I learn, is Aloysius Bertrand's "Gaspard de la Nuit" of 1842). But if something is to be made, a manual helps, and this book is it.

As these things go, it might not be relatively long  the Grove Dictionary of Music and Musicians (2001 edition), for example, runs to 28 more volumes than this one  but you'd think twice before taking it on holiday, and if you dropped it on your foot you'd need a Dyson vacuum cleaner to clear up all the fragments of bone. Pushing 1,500 pages, excluding index, and, boasts the back cover, containing more than 1,000,000 words; I am inclined to take the back cover at its word.

The previous edition of this reference work, which came out 20 years ago, is a slim volume by comparison. Its preface, too, had a certain succinctness: "This is a book of knowledge, of facts, theories, questions, and informed judgment, about poetry," it began. This edition is somewhat drier, but places itself more firmly in the academy: "Poetics, the theoretical and practical study of poetry, is one of the oldest disciplines in the west, one of those founded by Aristotle along with ethics, logic, and political science."

It is brave of the editors to set out their stall in this fashion, and might go some way to soothing the tempers of those minded to go hrrumph when contemplating the existence of three separate entries dealing with poetry written by or about those who love people of the same gender. (Or one entry on cowboy poetry. Well, why not?) If there is one thing we learn above all else from this book, it is that poetry is something people do everywhere, and have been doing ever since there was language.

Being almost half as long again as its predecessor, the fourth edition might be said to be suffering from a kind of university-driven inflation, of the kind which would intimidate or alienate the general reader  the very list of contributors attests to a whole load of universities whose existence may be news to you. (Although all are respectable, and the list is perforce weighted towards the Ivy League  which is nothing to complain about.) But then there has been an awful lot of book-larnin' even in the last two decades, and if the last entry in the 1993 edition, Zulu poetry, simply redirected us to "African poetry", is there really anyone who is going to complain that it now has its own entry? We even get to read some: "Umahlom'ehlathini onjengohlanya,/Uhlanya olusemehlwen' amadoda"  "He who armed in the forest, who is like a madman,/The madman who is in full view of the men," an example, we learn, of Parallelism  a technique that exists in poetry from the Kalevala to the Ugarit poets of ancient Syria to  well, whenever. And if the entries themselves, being restricted to information tend to be dry (although it is lovely to be reminded in the relevant entry that one of the uses of allusion, according to Christopher Ricks, is to assuage the poet's loneliness), it is because they have to be; there is much to say (I also like the way certain words are abbreviated  hist, contemp, Gr, etc  just as they would have been in your school notes.

TS Eliot scorned the idea that poetry could save us, saying that it was like thinking the wallpaper could save us when the walls had crumbled; but if you want to know how that poetic wallpaper gets hung, in a fashion that makes you realise there is an impulse to poetry in us that is universal, there is really no better book than this.