Black Gull Books, N2 8AG
Sections include fiction, children's books, cookery, travel, art, history, and books of Irish interest. There's no separate London section; London books are mixed in with the rest of the travel section, and seemed quite sparse on Kake's March 2009 visit, though the stock does change regularly.
Each book is priced individually in pencil, and the prices on the ones I looked at ranged from £3 to £15, though doubtless there are others outside this range.
Accessibility: One step up to get in, but there's a reasonable amount of room to move around inside.