It's worked multiple ways for me, depending on the show. My most current form is a mainly-electronic book. I keep on hard copy my script, emergency info, contact info, and run sheets; the rest is completely electronic. I always keep a USB flash drive in my book when it is currently in production, and burn a hard CD for it when it is done (don't worry, I have multiple back-up locations for all my files, just in case).
For previous shows, I have had two books, a "show bible" for all the relevant information and a "script bible" for my script and on-hand information. I've also just used one book for all information. It really does depend on the show, size of script, etc. However, I usually put my script right in front behind contact info, just cause It's easier to turn pages that way