The Elements of Geometry by Euclid

"There is no royal road to geometry."

For over 2,000 years, this was the primary textbook for mathematics in the Western world. Euclid compiled the knowledge of ancient Greece into a rigorous system of axioms, proofs, and logical deductions that remains the gold standard of mathematical reasoning today.

Abraham Lincoln used to read Euclid to train his mind in logic. It is a thing of pure, abstract beauty. From the construction of an equilateral triangle to the proof of the Pythagorean theorem, it teaches you how to think clearly and irrefutably.