What Is Mathematics 
BOOK 

Probability Through Problems 
PDF ; BOOK 
Best introductory probability book out there. Learn by discovering! 
Essential Linear Algebra with Applications: A ProblemSolving Approach 
BOOK 
Learn linear algebra through problems. 
Principles And Techniques In Combinatorics 
BOOK ; PDF 
Beautiful book: simple, clear explanations, no filler, challenging problems 
Putnam And Beyond 
BOOK 
Lots of challenging math problems from various competitions. 
Game Theory Evolving 
BOOK 
Good introduction to evolutionary game theory, with example problems. 