<human>:  How many books did the 5th century mathematician Euclid write?
<bot>: Euclid wrote 13 books, which are called the Elements. The first book, called Book 1, is all about definitions, and the other 12 books are all about proofs. The first book is really important because it contains the first axioms for mathematics. The first book is considered to be the most important book in the history of mathematics.