The best GRE prep book is the one that will help you achieve the score you need on the GRE. There are a number of different prep books available, and it is important to find one that is well-suited to your learning style and needs. Some students prefer prep books that offer extensive practice questions and detailed explanations, while others prefer concise review materials that can be easily referenced during studying. Ultimately, the best GRE prep book for you is the one that will help you reach your desired score.