Who doesn't want a secret door bookcase? We have a space in our living room that I've been planning to conceal with just such a door for 10 years, but I've kept putting it off because doing something like this has to be done well. I wanted the carpentry to be of high quality and the mechanism to be rock solid, and I finally got the time, resources and hardware to make it to the standard I wanted.
This is a long instructable, because it covers not just the construction of the bookcase but also some other features - it has a unique locking mechanism based on a sliding book, multiple secret compartments inside the bookcase (aside from the big one behind it!), and it's fitted with electronics that generate some geeky but fun sound and light effects.