This is a wonderful book. Subtitled 'Nine Excursions in The Secret History of London', it's the psychogeographic diary of a brace of wanderers on the streets of our marvellous capital, armed only with notebook and camera. Iain Sinclair is interested in everything, from pulp writing to Nicholas Hawksmoor's terror and magnificence, from Lord Archer's perspective, from Alembic House, on a 'khaki Thames' to Year Zero graffiti ('the Tourette's syndrome ravings of an outwardly reformed city'). Maybe Sinclair is sometimes too interested in everything, but the enthusiasm with which he seizes whatever hoves into view is infectious.

This pair (he travels with snapper Marc Atkins) cover the waterfront (a river, literally, runs through it): parading from Hackney to Chingford; scouting the surveillance cameras atop Terry Farrell's temple of MI6 black-ops ('so many...they seem more of a design feature, artificial birds'); risking arrest on the City's so-called 'ring of steel'; then over to the East End to bury Ronnie Kray; and up into the skies to scrutinise the perfectly aligned bridges and the art collection in Jeffery Archer's luxuriously appointed penthouse. The book has no apparent purpose other than the self-imposed vectors it describes.

Sinclair is a novelist of the anarchic Left, described by the late Angela Carter as 'crazy, dangerous, prophetic'. No bones are made of his prejudices through these pages, not least on the second walk, 'The Dog & The Dish'. Hackney is a land of pit bulls ('a prick with teeth') and satellite TV ('a long distance heart attack, incremental cancers'). John Major visited here ('a bloodless apparition...an understudy for Gilbert and George').

For all his heart-on-sleeve politicking Sinclair is curiously conservative, sharing his city with Peter Ackroyd's occultist architectures and crepuscular magik. His city is underlaid by ley lines and sacred energies, fecund with possibility and inherent otherness. If we don't quite appreciate Sinclair's psychogeograpies, maybe that's because he doesn't fully understand them either: there is something defiantly instinctual here.

The book transforms in its final third. The physical gives way to the psychological and Sinclair finds himself travelling through an internalised London, with a compendium of writers and fabulous films. 'Cinema Purgatorio' is a superlative essay, taking in Sinclair's own flirtations with film-making, up through Patrick Keiller's remarkable fictionalised travelogue London, and into the brief times of Witchfinder General director Michael Reeves, a suicide at just twenty-five. (Did you know, for example, that Reeves made three films, not just the two usually attributed?) The last chapter, dedicated half and half to eulogising the late Derek Raymond and pricking the bulbous ego of PD James ('scene setting worthy of Enid Blyton'), is as delicious as irrelevant.

The story Lights Out tells is sometimes obfuscatory, sometimes so bound up in the author's own arcana that we see no chink between the bricks, but Sinclair keeps us as aboard as he does by sheer force of telling. In addition, there is enough sly humour to engage even as he dazzles: the Albert Hall is 'simply the poshest church in England...a Kensington scout hut with budget'; uniformed security personnel 'slightly too plump for their Star Trek leisure wear'; buildings 'so dull that you'd have to be out on license to notice them'. Sinclair is a real writer, his tome more than a history, a simple guidebook -- a corrupted novel, maybe, or a script for the greatest London movie never made; a madman's tongue lashings. Sepulchres to dog shit, prostitute cards to the Barings collapse, Lights Out For The Territory might just be the best book about London ever written.