I like this book on several levels. This book is the standard for what it is teaches for developing good quality documentation: it is easy to use as a reference, with headings and a table of contents that many other technical books would do well to emulate. It clearly explains how to write or techniques to follow to help improve your writing quality, along with the why. It's recommendations are immediately useful.

If I had to choose only one book to refer to on my bookshelf, I'd toss aside my style guides, and keep this nearby.