If you are new to technical writing, it would be hard to find a better primer than this one. Brimming with realistic examples and strong opinions (perhaps occasionally too strong?) about exactly what your content and style rules should be, this book crammed a lot of best practices into 541 pages. Even if "in real life" I'll never be able to apply all this great advice and still make my deadlines, I'm getting closer than I otherwise would have.
The book design and layout made me twitch, but that's just my pre-tech writing publishing maven talking, and I have to learn to ignore her now.