I'm in the middle of writing all of the user manuals and tutorial documentation, as well as sell sheet & PDFs for our new software, INSPIRE. So far, I have found some really valuable resources for getting refreshed on the joys of technical writing.

1. Mountain Dew
   
2. The Handbook of Technical Writing, Seventh Edition
   
3. Technical Writing 101: A Real-World Guide to Planning and Writing Technical Documentation, Second Edition
   

Dana: Check out "Content Critical" by Gerry McGovern. While not specifically about technical documentation, it's a great read about writing and giving your readers what they want and need.