Important textbooks for the biology olympiad

Check How To Prepare for the Biology Olympiad And Science Competitions by Martyna Petrulyte.

Also amazing books include:

Campbell Biology

Campbell Biology (11th Edition) (9780134093413): Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Jane B. Reece: Books

Life: The Science of Biology

Life: The Science of Biology (9781429298643): David E. Sadava, David M. Hillis, H. Craig Heller, May Berenbaum: Books 

Raven

Peter H. Raven: 9780077350024: Amazon.com: Books

Brooker

Genetics: Analysis and Principles (9780073525341): Robert J. Brooker Professor Dr.: Books