Style Guides

The Microsoft Manual of Style for Technical Publications is our house style guide, of course. It contains information specific to computer terminology and usage (e-mail, toolbar), and it also gives guidance for general writing (that vs. which, tone and the reader). The broad coverage makes it very useful as a single reference that I can direct writers to.
Our secondary style guide is The Chicago Manual of Style, which I rarely have to use. I'm glad I don't need it very much; I'm so used to our online version of the Microsoft guide that I find it difficult to locate items in the Chicago manual without a good deal of searching.
I also like to read general and popular style manuals to get an idea of the different points of view in the editing and writing world. Right now I am alternating between Torn Wings and Faux Pas by Karen Elizabeth Gordon and Lapsing into a Comma by Bill Walsh. Reading through these guides gives me a better idea of what I should look up in my official style guide when I see it the topics I edit.