I recommend the paper (series of class notes) Mathematical Writing, 1989, written by Donald Knuth ("Art of Programming" author, among other things), Tracy Larrabee and Paul M. Roberts.
Here is a description from Stanford:
This booklet records the class on Mathematical Writing led by Don Knuth at Stanford in 1987. Among the 31 lectures are guest appearances by Herb Wilf, Jeff Ullman, Leslie Lamport, Nils Nilsson, Mary-Claire van Leunen, Rosalie Stemer, and Paul Halmos.
We saw many examples of "before" and "after" from manuscripts in progress. We learned how to avoid excessive subscripts and superscripts. We discussed the documentation of algorithms, computer programs, and user manuals. We considered the process of refereeing and editing. We studied how to make effective diagrams and tables, and how to find appropriate quotations to spice up a text. Some of the material duplicated topics that would be discussed in writing classes offered by the English department, but the vast majority of the lectures were devoted to issues that are specific to mathematics and/or computer science.
It is helpful not just for mathematicians but scientists and technical communicators of all sorts.