Old Men in Love, By Alasdair Gray

Alasdair Gray, it seems, is unwilling to muck about with a good formula; but then again, none of his fans would want him to. From the very start, his 20th book plays the kind of good-natured, half-heartedly postmodern games that its author has been quietly enjoying ever since the genre-swallowing metafictional fantasia of Lanark sent him to the front of the class in the 1980s.

The introduction to Old Men in Love is provided by an American businesswoman called Lady Sara Sim-Jaeger, the heir and executrix of John Tunnock, a retired Glaswegian schoolteacher of murky habits. After his death in mysterious circumstances, Lady Sara goes through Tunnock's papers, which include an autobiographical sketch, a moderately salacious diary and a few (possibly plagiarised) drafts towards a novel, then hands the whole lot over to a local writer who – after providing a title, illustrations and marginalia – hopes to claw back some cash from Bloomsbury, "a highly successful firm that had done well out of J K Rowling's Harry Potter books".

Yes, Lady Sara's name is an anagram (Gray's middle name is James). This whole business of "text-as-found manuscript, invented by Scott ... afterwards plagiarised by Hogg, Pushkin, Kierkegaard, Dostoevsky and Gray" is lengthily criticised in an unnerving postscript to the novel: Gray likes to provide what he calls "critic fuel" at the end of his books, a practice bloggers generally call trolling. Old Men in Love is sophomoric at times, so-whattish at others, self-willed throughout – and a work of some genius.

John Tunnock lives alone in a 19th-century townhouse in Glasgow, occasionally accompanied by one of the "young things" he meets out on the streets and invites back home. Intercut with the scribbled and vaguely bonkers record of Tunnock's senescent disappointments, we find three parts of a historical narrative: the first set in Greece under Pericles, as a stonemason called Socrates shivers in the hills and wanders through the forum; the second describing the life of the morally fluid priest and painter Fra Filippo Lippi under the Medici in Florence; and the third drawing on diaries and period reports for an account of the Lampeter Brethren, a Victorian sect whose charismatic leader eventually announced himself as the Messiah.

There's also an extremely funny section describing Tunnock's upbringing by two socialist maiden aunts in Glasgow; a play presenting an alternate version of the trial of Socrates that Gray wrote for radio some years ago; a spoof letter from the historian Angus Calder to Tunnock; a version of Gray's newspaper article about the anti-war march in Glasgow in 2003; and much more.

Scottish nationalism, masturbation, curmudgeonly metafictional jokes, a wealth of beautiful illustration: if you like Alasdair Gray, this has it all. Old Men in Love may not be much of a novel, in the way that we traditionally speak of novels, and it's hard to argue that it's much of a non-novel, either, in the conventionally subversive sense. Whatever it is, though, no one else could have written it: and that alone should be reason enough to support Scotland's weirdest writer by going out and getting it forthwith.