Here's a good place to start...
http://www.amazon.com/Control-Systems-Live-Entertainment-Huntington/dp/0240803485
It can be a tough business to get into because reputation and connections are critical. Not to say impossible, but it's a tough shell to crack. I stumbled into it many years ago, when it was a different era of touring production. The 'wild west' so to speak, before it went all corporate.