I second Backstage Handbook and the Stage Rigging Handbook. I also like "
Practical Health and
Safety Guidelines for School
Theatre Operations" By Dr. Randall W.A. Davidson (aka. Dr. Doom). It is not the cheapest book, but I found it to be very informative, very dense with lots of good information and photos. Don't let the title fool you, everyone can benefit from it equally, not just schools. It is nice if you are looking to convince non theatre administrators (either a board of directors, school administration,
etc.) of the risks that can be found in a theatre.
For sound, I enjoyed the Yamaha "Sound Reinforcement Handbook" by Davis and Jones. The material is a bit dated, but it does a good job of explaning the theory and mathematics behind audio.
~Dave