This is where most of my "technical" books end up - I like having them right behind me while I'm working. The most useful, lately, has been PHP Functions Essential Reference, from New Riders. I've referred to it many times while working on this site, and also at work. (In fact, I bought one for work a few months ago and bought this copy for home; it's a little too heavy to drag back and forth all the time.)

At the bottom of this list you'll see Home Buying for Dummies, the only "For Dummies" book I've ever owned - or hope to own. It was very useful during the home buying process, but the title really bugs me.