There are alot of books in the market for olympiad preparation.
Following are the best referred books for the preparation of Maths Olympiad:
1. An Excursion in Mathematics
Editors: M. R. Modak, S.A. Katre and V.V. Acharya (Bhaskaracharya Pratishthana, Pune, 2008).
2. Problem Primer for the Olympiads
C.R. Pranesachar, B.J. Venkatachala and C.S. Yogananda (Prism Books Pvt. Ltd., Bangalore, 2008)
3. Challenge and Thrill of Pre-College mathematics
V. Krishnamurthy, C.R. Pranesachar, K.N. Ranganathan and B.J. Venkatachala (New Age International Publishers,  New Delhi - 2007).
4. Problem Solving Strategies
Arthur Engel (Springer-Verlag, Germany, 1999).
5. Functional Equations
B.J. Venkatachala (Prism Books Pvt. Ltd., Bangalore, 2008).
6. Mathematical Circles: Russian Experience
Fomin and others (University Press, Hyderabad, 2008).

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