I couldn’t settle on just one book for the first in my list of outstanding books, and so I selected two! Anyone who has ever taken trigonometry or calculus will appreciate these books.

The books Trigonometry Refresher and Calculus Refresher by A. A. Klaf are timeless classics on the subjects of trigonometry and calculus. The author uses a question and answer technique in these books to refresh your memory of trigonometry and calculus. He asks the questions you should have asked when first taking trigonometry and calculus and then answer those questions. This approach is especially helpful as it gives you a thorough understanding of the subject without having to work laboriously through the thick textbooks you used to first learn trigonometry and calculus.