I picked up a copy of Knuth's mathematical writing notes when I was a graduate student in computer science (really not that long ago, actually). This book is a great starting point for new technical writers, and it is a great supplement for more experienced writers that want to polish their craft.
This manuscript is a series of Knuth's notes that comment on a variety of topics related to making technical writing both precise and readable. Admittedly, some are more useful than others, but everything considered the useful material is definitely worth the cost.
Knuth provides a lot of do's and don'ts for people who do math-heavy writing, many of which stick with you after one read. I could see how this book could be a useful resource for grad students, post-docs, or researchers/teachers in a number of fields. If more effort was put into refining the material I think that it could have been a much more valuable and definitive book on the subject (I'm not aware of any others, but more texts like this are needed). Still, it is well worth the money.