Mathematical writing by Donald E. Knuth, Tracy Larrabee, and Paul M. Roberts. Reading the first chapter is a great way to get in the right frame of mind for writing.
On Writing Well by William Zinsser is a book about writing non-fiction in general. One of my favorite books on writing, full of excellent advice.
As is The Elements of Style, by William Strunk Jr. and E.B. White. (In fact, read E. B. White to see how good simple prose can be, and how devilishly difficult to get right.)
Kurt Vonnegut's 8 Basics of Creative Writing are just as essential to math writing.
In fact, check out tips from other masters on the same website. (Cf. Marshall McLuhan: "Anyone who makes a distinction between entertainment and education doesn’t know the first thing about either.")
For math papers (but with great advice about technical writing more generally): How to write Mathematics by Paul Halmos, and How to Write a Clear Math Paper by Igor Pak, among others.
Many of my suggestions for giving talks have natural analogs for writing.