The Cambridge University Press bookshop in Cambridge, UK is a lovely shop with eightnine bookshelves of mathematical books. There's approximately one bookshelf each for algebra, analysis, number theory, combinatorics, methods, geometry, probability & statistics, and recreational mathematics. They stock only books they publish themselves (of which there are several thousand mathematical ones), including a good selection from these series: Cambridge Mathematical Library, Cambridge Studies in Advanced Mathematics, the LMS Lecture Note series.