Useful links

TheBookGuide
A comprehensive and frequently-updated guide to sources of secondhand and antiquarian books throughout the UK.
Randomness Guide to London: bookshops category
Made by the same people as this site, with writeups that go into more depth. Not comprehensive; only covers the bookshops we've actually been to.
londonbooks.tumblr.com
Photos and brief writeups of London's most interesting bookshops.
Londonist: Biblio-Text
A series of articles on London's independent bookshops.
The (other) London Bookshop Map
Map showing 100 independent bookshops in London, with brief writeups. Differs from this site in that it's selective rather than comprehensive. Also available in paper form.