The official textbook of the USA Biology Olympiad competition is Campbell's Biology. Honestly, we love this book. It covers a large majority of what you need to know for the competition in a pretty clear format. It is entirely possible to become a top 20 National Finalist with only using this book (Varun did).

If you feel like you have Campbell's down pretty well, Raven Biology of Plants is your next stop. While this book is a bit more dense than Campbell's it helps with problems in molecular biology, plant biology, systematics, and ecology.

Now for the (in)famous Molecular Biology of the Cell by Bruce Alberts. This book is extremely helpful for the more detailed molecular biology questions, though it is often a bit overboard. While you can read it, we recommend you use it more as a reference guide, from which you read topics you're weak on.