Here are other books that I found to be very useful:

Technical writing – general

• The Complete Idiot’s Guide to Technical Writing by Krista Van Laan, Catherine Julian, JoAnn Hackos
• Technical Writing 101: A Real-World Guide to Planning and Writing Technical Documentation by Alan S. Pringle, Sarah S. O’Keefe
• Technical Writing: A Practical Approach by William Sanborn Pfeiffer
• Professional and Technical Writing: Problem Solving at Work by George E. Kennedy, Tracy T. Montgomery
• Developing Quality Technical Information: A Handbook for Writers and Editors (2nd Edition) (IBM Press Series–Information Management) by Gretchen Hargis, Michelle Carey, Ann Kilty Hernandez, Polly Hughes, Deirdre Longo, Shannon Rouiller, Elizabeth Wilde

Technical communication

• Technical Communication by E. Burnett
• Designing Visual Language: Strategies for Professional Communicators (Part of the Allyn & Bacon Series in Technical Communication) by Charles Kostelnick, David D. Roberts

Software documentation

• Writing Software Documentation: A Task-Oriented Approach (Part of the Allyn & Bacon Series in Technical Communication) by Thomas T. Barker
• The User Manual Manual: How to Research, Write, Test, Edit & Produce a Software Manual (Untechnical Press Books for Writers Series.) by Michael Bremer

Style-related

• Technical Writing Style (Part of the Allyn & Bacon Series in Technical Communication) by Dan Jones
• Elements of Style by Strunk & White
• Microsoft Manual of Style for Technical Publications Third Edition (Paperback) by Microsoft Corporation
• The Chicago Manual of Style 13th Ed
• The Gregg Reference Manual – Wililam J Sabin
• Style and Readability in Technical Writing: A Sentence-Combining Approach by James Degeorge

User-focused

• Untechnical Writing – How to Write About Technical Subjects and Products So Anyone Can Understand (Untechnical Press Books for Writers Series) by Michael Bremer
• Observing the User Experience: A Practitioner’s Guide to User Research (Morgan Kaufmann Series in Interactive Technologies) (The Morgan Kaufmann Series in Interactive Technologies) by Mike Kuniavsky
• User and Task Analysis for Interface Design by Hackos, JoAnn & Redish, Janice C.
• Human-Computer Interaction by Alan Dix, Janet E. Finlay, Gregory D. Abowd, Russell Beale

Doc-planning

• Managing Documentation Projects by Jo Ann Hackos
• Starting a Documentation Group: A Hands-On Guide by Peter J. Hartman

Editing

• Technical Editing by Carolyn Rude
• Editing Technical Writing by Donald C. Samson

Grammar

• Eats, Shoots, and Leaves by Lynne Truss
• Words by Bill Bryson
• Elements of Technical Writing by Gary Blake & Robert W. Bly
• Understanding and Using English Grammar – Betty Schrampfer Azar

Dictionaries

• Merriam Webster’s Collegiate Dictionary (ISBN: 0877798087)

Online communication

• Standards for Online Communication – JoAnn T Hackos
• Writing Online Help by Lindsey

Usability

• Don’t Make Me Think: A Common Sense Approach to Web Usability by Steve Krug
• Paper Prototyping: The Fast and Easy Way to Design and Refine User Interfaces (The Morgan Kaufmann Series in Interactive Technologies) by Carolyn Snyder
• A Practical Guide to Usability Testing by Joseph S. Dumas, Oracle Corporation, and Janice C. Redish, Redish & Associates

Single sourcing

• Single Sourcing: Building Modular Documentation by Kurt Ament

Indexing

• Indexing: A Nuts-and-Bolts Guide for Technical Writers (Engineering Reference) by Kurt Ament
• Software for Indexing by American Society of Indexers (Corporate Author), Sandi Schroeder (Editor)

Software-development approach

• Agile Documentation: A Pattern Guide to Producing Lightweight Documents for Software Projects (Wiley Software Patterns Series)
• Understanding and using English Grammar – Betty Schrampfer Azar
• Standards for online Communication – JoAnn T. Hackos and Dawn M. Stevens
• The Chicago Manual of Style 13th Ed
• Managing Documentation Projects by Jo Ann Hackos
• The Gregg Reference Manual – Wililam J Sabin
• Human Factors for Technical Communicators – by Marlana Coe