I will consider expanding my article "Why do mathematicians re-prove theorems?" into a book-length study of the roles that new proofs of old theorems play in mathematical practice. I will consider alternative proofs of various well-known theorems (the fundamental theorems of arithmetic and algebra, the infinitude of the primes, the Pythagorean theorem, the law of quadratic reciprocity, the prime number theorem, etc.) and solicit suggestions for further examples to study.