Over Christmas break, I will spend some extra time studying for USABO, or Biology Olympiad. Even though I am currently taking AP Biology, I quickly realized that AP Curriculums in most schools don’t completely cover the curriculum required for similar competitions for high-school students. After a little research and personal experience, I found the following list of books the most helpful:
Campbell Biology (Any edition after 8th)

This series of textbooks has historically been a must-have if you are studying for USABO or any similar biology-related competition. It is by far one of the most comprehensive and time-saving resources I have found. I currently have the 11th edition, which is not much different from the 10th or 12th editions content-wise. The price of a textbook from this series ranges from 60 dollars to a whopping 300 dollars (if you are considering buying a textbook). Still, many libraries offer this textbook, making it accessible if you would like to use it.
Anatomica: The Complete Home Medical Reference

This book is one of the most comprehensive human anatomy and physiology resources I’ve used. Although not for the faint of heart, with highly detailed diagrams of internal organs and organ systems, I found this book highly helpful for the human A&P class I am currently taking at school. Furthermore, the book contains some more information detailing diseases and disorders affecting each one of the body systems it covers, making this book useful for those who want to learn a bit about medicine.
Overall, if you really love biology or science classes in general, I would recommend both these books as a more comprehensive way to learn at a higher level about biology-related subjects!