Style guides for software documentation

TechScribe recommends the following style guides:

Read Me First! A Style Guide for the Computer Industry. Sun Technical Publications, 2003. ISBN 0-13-142899-3. Read Me First! is the best practical style guide that we know about.
Microsoft Manual of Style, Fourth Edition, 2012. ISBN 978-0-7356-4871-5. See 'Microsoft Manual of Style: a review'.
Apple Style Guide explains the style and the use of Apple terms, including user interface terms (https://help.apple.com/asg/mac/2013/ASG_2013.pdf). The Apple Style Guide gives guidelines about glossaries, indexes, and writing for an international audience.
The Missing Manual Authors' Guide (http://missingmanuals.com/author_guide_v3.pdf) is for technical authors who write for the Missing Manual series. The style guide contains useful general advice.

General style guides

Other excellent style guides are available, but they are for professional technical writers. We think that the style guides in the list above are the most suitable for people who are not professional technical writers, but who write software documentation as part of their work.

For excellent general advice, see:

The Oxford Manual of Style
The Chicago Manual of Style
The Elements of Style, William Strunk (free to view, www.bartleby.com/141/)