The epicentre of this intellectual earthquake is, as it has always been, the Charing Cross Road. Here, Victorian and Edwardian shab has been effortlessly maintained in among the glaring lights and signs of nearby Soho.
My first forays to this curious, sleepy world began in the early 1970s, arriving on my bicycle from Wimbledon at the age of 15. Not a terribly good idea nowadays, so I recommend beginning at Charing Cross Station, or walking over the river on Hungerford Bridge from Waterloo. If you come from the north or west, it's probably better to take the Tube to Leicester Square and start there.
If you can avoid it, don't bring a big bag. Most of the bookshops were designed with books, not punters, in mind.
Those arriving at Charing Cross, or by foot from Waterloo, have a treat on Saturdays, much the best day for a visit. A collectors' fair resides in a claustrophobic concrete and brick bunker under the station, just outside the entrance to Embankment Tube. Here, various individuals display their wares: stamps, medals and Roman coins. The latter, recognisable and with an emperor's face, can be had from 50p upwards. They're genuine, too. No one bothers to make fakes. The Romans made quite enough authentic ones.
But the real action starts a few yards north. Whip up past St Martin- in-the-Fields and cut round past the statue of Edith Cavell and along Charing Cross Road until you see the entrance to Leicester Square Tube station. Just beyond, on the corner of Great Newport Street, it's time to get your wallet out.
Rule number one is that the shabbier the shop and the damper the basement, the better the bargains and the selection.
First stop is Number 48a - Quinto, with its glossy green decor and interior gloom. An extraordinarily long list of helpful directions to various London local spots nailed to the door shows that the assistants here spend (or would otherwise spend) a lot of their time directing 7ft-tall Dutch students with backpacks the size of an Apollo astronaut's life-support system to the Leicester Square loos.
But the shop is a creaking goldmine of unexpected possibilities. Two pounds bought me Target Germany, the wartime HMSO edition of the Eighth Air Force's exploits between 1942 and 1943. Downstairs in the airless, cobble-floored basement I chanced on a 1723 Chronological Historian by "Mr Salmon", still in its original binding and completely usable, for pounds 22.
The best thing about non-specialist bookshops is that great books cost little more than crummy ones (depending on your point of view). Next door's Francis Edwards is a military specialist, and the prices reflect this. It's not unknown to find the same titles in Quinto for half the price.
But don't spend all your cash at once. The shops beyond are mines of potential. Next stop is Charing Cross Road Bookshop, a few doors along. The basement is the place to head for, but like most of these shops, you must leave your bag at the till. Not a great deal of care is taken of them, so don't leave anything valuable inside. I found a first edition and first printing of Richard Hillary's Battle of Britain classic, The Last Enemy (1942) for pounds 3, and a beautifully printed, folio-sized 1899 Victorian atlas of the world for a tenner.
Next door's Henry Pordes Books is a classier establishment, where the secondhand books are dear and the remaindered art, antiques, travel and history stock cheap. At pounds 5.99 I was hardly going to leave a brand-new copy of Antonia Fraser's 1997 The Gunpowder Plot behind (jacket price pounds 20).
Any Amount of Books, neighbour to Henry Pordes, is part of the Charing Cross Road Bookshop series and offers exactly the same cluttered fare of Everyman books, paperback novels and the extraordinary archaeology "classic" by C W Ceram called Gods, Graves and Scholars. This astonishing title must have been produced exclusively as bookshop ballast, like Churchill's History of the English Speaking Peoples. No secondhand bookshop ever has fewer than two copies and I have never seen one in someone's house. I have never seen anyone buy it either. Who reads it? Who has read it? Well I did once, when I was 12, but I have resisted it ever since.
The next few shops, Silver Moon Women's Bookshop, Shipley Art, Zwemmer Design, and Al Hoda are all specialist emporia and need not delay the impulse book-buyer. Whip on past Cambridge Circus, if you dare cross Shaftesbury Avenue, and head past the new Blackwells.
On the other (west) side of the road, the fluorescent orange decor of Lovejoys shines like a beacon. No antiques-books centre, but actually a sex shop, plain enough from the raincoated men who enter, glance momentarily at the innocuous stock and scuttle downstairs for more lugubrious fare. But upstairs, at street level, is a jolly good selection of cheap remaindered literature, history and militaria. I picked up a new copy of Flying Start, the reminiscences of Hugh Dundas, one-time chairman of Thames TV and former Battle of Britain pilot, for pounds 2.
If you carry on up this side of Charing Cross Road you will inevitably hit Foyle's and its idiosyncratic collection of salespersons, whose variable command of English and occasional familiarity with the stock make for a unique book-buying (or not-buying) experience.
There are scattered selected secondhand books in among the stock, but they tend to be priced up on the grounds of rarity. Getting to know this shop is a career in itself, and you should set aside a day devoted to it alone.
So cross back again to the east side and go into the very last store before Centrepoint, handily opposite an Ann Summers sex shop. Called Bookcase, the fun is downstairs past the usual collection of remaindered "art" (= nude) books. This is an excellent secondhand basement with a terrific archaeology and history section, though not desperately cheap, and also large quantities of new children's books. A few brand-new volumes of the outstanding modern edition of Pepys's Diary can be had here for a tenner (normal price pounds 35), so if you're one of the many people who bought the earlier volumes when they came out and missed the Companion and the Index, then here's your chance.
This part of London is especially revolting. It's the unpleasant end of Oxford Street, distinguished by peeling posters, filthy pavements, huge black puddles by gutters, crateloads of fast food-wrapper rubbish and vast numbers of people handing out English-lesson advertisement cards.
So perhaps it's time to hit the Tube at Tottenham Court Road and escape. A fistload of proper chips can be had at Dionysus Fish Bar (on the corner of Oxford Street and Tottenham Court Road) before you go, and they make a pleasant reward for all the foot-slogging.
There are plenty of other bookshops in the area. Unsworths in Gower Street is the mother of all remaindered-book bookshops.
Close by in Great Russell Street, on the corner with Gower Street is Museum Books, packed with all sorts of archaeology and Egyptology classics, and just past it is the Antiquarian Book Arcade.
Bloomsbury Bookshop in Bury Place, further along Great Russell Street past the British Museum, has a congested collection of secondhand academic books.
Finally, Skoob Books in Sicilian Avenue, between Bloomsbury Way and Southampton Row, has something of everything and is close to Holborn Tube. Some of these shops are open on Sundays but don't count on it. Saturday is the day and don't start before 10am, better still 11am