Knuth, mentioned in another answer, is trustworthy and a good author. The book mentioned by the OP maybe be useful to beginners, but I don't agree with many of Krantz's (the author) opinions about writing. In general, people have various opinions and tastes about writing, so it's good to see a spectrum of opinions and determine what approach fits for you in a particular situation. And now that you've started thinking about these things, read mathematics critically so you can build up examples of what you think works well and what doesn't.
Anyway, some other references are:
Terry Tao's advice on writing, which includes many links to pieces written by other authors
Milne's tips for authors, which includes a reference to Krantz's book and an essay of Hersh)
It might also not be a bad idea to read general advice about writing (not just mathematical/technical writing).