Euclid's "Elements" is an extremely popular mathematics textbook; this book provides a comprehensive basis for understanding the field of geometry. "Calculus" by Spivak is widely praised by reviewers as one of the best modern textbooks for learning single-variable calculus.
Spivak's "Calculus" is a popular introductory textbook for learning single-variable calculus. Some Amazon reviewers cite the exercises in "Calculus" as being somewhat monotonous and overly similar, but overall, this book is highly reviewed, receiving an average rating of 4.5 stars. Popular textbooks directed toward other fields of mathematics include "Godel, Escher, Bach: An Eternal Golden Braid" by Douglas Hofstadter and "Algebra" by Gelfand/Shen.