The Microsoft Manual of Style for Technical Publications by Microsoft

This is a good resource for technical communicators and software developers working with the Windows operating system. This book provides an accessible guide to documentation style practices used by Microsoft's product groups.  The guide's scope includes the range of desktop software and hardware that are appropriate for Windows. The book also includes a disk with a Windows online help file containing style guidelines that the user can use as an online style reference.
 

Read Me First! : A Style Guide for the Computer Industry by Sun Technical Publications (Editor)

This book is Sun's take on everything you need to know about documenting computer products. It covers everything from basic punctuation and style pointers to legal guidelines and creating a documentation department. It also covers writing for an international audience. The book includes a CD ROM with FrameMaker templates including paragraph and character tags, for easy book creation using Sun's document design. Note: This book won a 1998 Distinguished competition award from the Society for Technical Communication (STC) in the Books category.
 

Chicago Manual of Style
by the University of Chicago Press

This is the essential style manual used by writers, publishers, and publications departments worldwide. The 14th edition of the manual, published in 1993, is a great improvement on their last edition published in 1982, with nearly 200 additional pages reflecting significant changes in style, usage, procedure, and technology.
 

The Elements of Style by William Strunk Jr., E.B. White, Charles Osgood, Roger Angell

Short, sweet, and to the point, this masterpiece in the art of writing clear and concise text promotes basic concepts such as using active voice and omitting needless words. This book will improve any kind of writing.

Managing Your Documentation Projects
by Joann T. Hackos

Managing technical publications can be quite challenging. This book provides steps to start, maintain and complete documentation projects in a timely, high-quality fashion. The book includes templates, checklists, sample reports, and B&W illustrations.