- William Dunham, Journey Through Genius: The Great Theorems of Mathematics.
This concise book has been used as a textbook in the past for MA 3513. It is fairly short and readable, although somewhat less comprehensive than Stillwell's book.
- Carl Boyer, A History of Mathematics.
A somewhat older treatment of the history of mathematics, which could supplement Stillwell's book.
- Imre Lakatos, Proofs and Refutations.
This beautiful book is perhaps more philosophy than history. It presents a thinly fictionalized version of how the historical development of the Euler-Poincaré characteristic of a sphere (and more general spaces). We won't have time to dive into this book. I highly recommend it, but do note that it is somewhat more mathematically sophisticated than Stillwell's book.
- Martin Aigner and Günter Ziegler, Proofs from the Book.
Paul Erdős was one of the more prolific and influential mathematicians of the 20th century, and also one of the more idiosyncratic. He had a religious belief in the Book of perfect proofs of mathematical theorems (perhaps maintained by God, see the linked article on Erdős).
The book of Aigner and Ziegler, partly in honor to the memory of Erdős, tries to collect some proofs that are so beautiful that they may be perfect. It often discusses the development of these proofs, which includes significant aspects of mathematical history.
has a pleasantly readable overview article on the History of Mathematics