TITLE: Preparing for Mathematics Olympiad
QUESTION [16 upvotes]: I am preparing for Mathematics Olympiad , can any one suggest me some books to prepare for olympiad ? The topics that usually come up involve: 

congruence modulo $n$, 
inequalities ,
number system, elementary number theory, etc.

Please help me!
Thanks
Kushashwa

REPLY [3 votes]: These following books are good:

Problem Solving Strategies by Arthur Engel
Mathematical Circles by Dmitri Fomin et al
The USSR Olympiad Problem Book by D. O. Shklarsky et al
Others books By Titu Andreescu and Isaak Yaglom