Campbell's biology is a fantastic general biology textbook and is highly recommended for this competition and the USABO. This textbook is a great introduction for new students, and we recommend that all students read this book.
Fox Human Physiology
Fox Human Physiology is a good textbook for those who would like to learn more specifically about human anatomy/physiology. Many concepts in this textbook are also applicable to animal physiology.
Ravens Biology of Plants
Raven's Biology of Plants is a great textbook for those interested in learning more about plants. It explains many concepts in a manner that is easy to comprehend.
Molecular Biology of the Cell
Molecular Biology of the Cell is a great textbook for learning more about cell and molecular biology. It explains many complex concepts and is very thorough and detailed.
Lehninger's Principles of Biochemistry
Lehninger’s Principles of Biochemistry is a textbook oriented towards teaching chemical biology and biochemistry. Many chemistry concepts are introduced and explained within the context of biology.
Griffiths' Introduction to Genetic Analysis
Griffith’s Introduction to Genetic Analysis is a thorough textbook for those interested in learning more specifically about genetics. It covers many useful concepts and helps one understand them intuitively.