These books are very useful for Mathematical Olympiads.
Arthur Engel - Problem Solving Strategies
E. J. Barbeau - Polynomials
Titu Andreescu, Bogdan Enescu - Mathematical Olympiad Treasures
Loren C. Larson - Problem solving through problems
Dusan Djukic, Vladimir Jankovic, Ivan Matic, Nikola Petrovic - The IMO Compendium
