Books
I don't recommend many books on writing, because I think that writing is best done by a combination of two things: reading a lot, and practicing constantly. That said, there are a few standouts, and I've collected them here. In time, I may expand the list to include some more niche books that are relevant for certain writing specialties.
Microsoft Manual of Style for Technical Publications
The definitive guide to writing software documentation. There's a reason just about every technology company uses it. Every technical writer should be familiar with its contents. There are times when Microsoft gives an electronic version away for free, but they are few and far between.
It is indeed "comprehensive, concise, and logical", as one Amazon reviewer put it, and I've used bits and pieces of it to create internal style guides for specific uses. A must-have for all technical writers.
Stephen King's On Writing: A Memoir of the Craft
This book is written for writers of fiction, but the lessons are applicable to nonfiction writers as well. In terms of mechanics, it draws a great deal of advice from the third book on this list. However some writers may find the editing examples useful, and King talks quite a bit about being concise, probably the single area writers universally struggle with.
Best of all, this book can be read on the beach on or an airplane. It's not a workbook by any stretch of the imagination.
Strunk & White's The Elements of Style
The classic. Perhaps the most famous phrase contained within is "Omit needless words" (Chapter 13). Words all writers should live by.
Older versions of this classic text are now in the public domain, which means you can read/download the text for free (example). Perhaps one of these days, I'll get around to formatting the book properly and making it available as a PDF, .epub, and/or put it up on Lulu so people can order a bound version for a few dollars. In any event, it's cheap, and it's a book that all writers and aspiring writers should have in their collection.