Problem books in higher mathematics Are there any problem book series on different topics with proper solutions? I have already found some in analysis (Problems in Mathematical Analysis 1-3) but not any in less popular topics.

Try "Contests in Higher Mathematics: Miklos Schweitzer Competitions, 1962-1991" - it's a book that contains problems (and their solutions) from the hardest higher mathematics competition (harder than the Putnam and the IMC). The contestants are given a week to solve the problems, and they can consult literature.
The topics "range from algebra, combinatorics, theory of functions, geometry, measure theory, number theory, operator theory, probability theory, topology, to set theory". Beautiful and engaging problems, written by Hungrian mathematicians, some of them well-known.
