SAT II Biology
SAT II Biology is split into Ecological and Molecular biology. These are both relatively easy if you're already preparing for AP Biology. If you are preparing for one of these tests, some good books are the Princeton Review and Barron's.
While there are many great books for AP Biology, our favorite is Campbell's Biology. If your class is already using this book, then great! If not, you should probably just use what your class is using for cohesiveness. However, if you are home-schooled, self studying biology, or in honors, but wanting to take the AP test, this is a great book to read!
USA Biology Olympiad
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.
If you have any other competitions you'd like advice on, please send us an email. We just might not have heard of the competition you are studying for, though we can take a look at some practice tests and see if they fit within our range of experience.