"Mathematical Writing" by Donald E. Knuth, Tracy Larrabee, and Paul M. Roberts
This is a report based on a course given at Stanford University in 1987.
I stumbled on a pdf version of this classic by D E Knuth. Gives you many rules of clean and clear mathematical writing. It borrows many ideas from the classic book of style by Struk and White. This is a must-read for all those who (like me) use maths for making a living. Wish my maths teachers had read this book before teaching me maths.