TITLE: Research-level mathematical bookstores
QUESTION [30 upvotes]: I'm interested in compiling a list of bookstores around the world that stock a good selection of high-level mathematical books. The aim is so that a mathematician travelling, or on holiday, can easily check the list to see if there is a book store nearby that she or he could happily spend an hour browsing in.
Some particulars:

It must be a physical bookstore.
It should have a good selection of research level books. This criterion is designed to exclude stores that stock primarily popular books and undergraduate textbooks, such as some university and chain bookshops.

REPLY [10 votes]: In Paris, there's Gibert (26-34 Boulevard Saint-Michel). It's a giant bookstore with several floors of books including one whole floor devoted to scientific books. A sizable part of that floor is dedicated to maths, and there are enough books that they're sorted by subject (number theory, algebra, analysis, ...). During Springer's yellow sale, there's a big table overflowing with cheap Springer books, including many tomes of Bourbaki. If you go, there's a high risk of running into somebody you know.