How to Write Mathematics
Norman Earl Steenrod, How to Write Mathematics. 0-8218-0055-8.
A collection of essays on mathmetical writing, of which Halmos’ is the most well-known and also available online.
The two other essays in this collection are not bad at all, but they are not as good as Halmos’ and reading the PDFs found online is a sufficient for getting the most out of this book. Also, note that the target of this advice is for monographs, books and the like. While most of the wisdom does apply to any kind of mathematical writing, it does presume that the writer is experienced enough to consider writing a book, which is a high order for someone who is just interested in improving their mathematical writing.