It wasn't until I was in high school that I discovered those doors were real. In one of my friend's houses there was a section of wall paneling that would swing open when you pushed on it in a certain way. Turns out the hidden cupboard behind it was used to hide booze. There were similar doors in the backs of closets that led into the closets in neighbouring rooms. My friend wasn't sure just what sort of a place his house had been, but it made for some interesting high school parties.
Stashvault is a site dedicated to hidden doors, vaults and passageways amongst other secretive building ideas. The article I came across was a conglomeration of other sites explaining how to build, or offering to build bookcase doorways. The trick it seems is balance and clearances. Check out the full article here.