i'll second church st stoke newington. but get in quick because the rent has tripled in that area recently. the latest casualty is totem records. you may still be able to get some bargains there before it closes up for good - follow the flocks of dealers like vultures.
back on topic: just off old street, above the foundry..
Bookartbookshop 17 Pitfield St. London N1 6HB
more artists books and editions than theory, and very very small, but a treat and worth mentioning.
these two popped up in my websearch for bookartbookshop:
Index Books Peter Gidal 28 1/2 Lansdown Crescent London W11 2NS
William English 48 Galway House Radnor St. London EC1V 3SL
do they mean anything to anyone?