Not that it happens often, but every now and then I actually clean off my desk. It’s one of those things that I sort of know intuitively that I work much better with clean desk.
This book on regular expressions, one of the Pocket Guide series by O’Reilly, is one that tends to always be on my desk though no matter how clean I get it. Heck, it’s even pretty much set to permanently open to the PHP section because that’s what I use most often.
I don’t have many books that get as much use as this one. It’s even getting to a point where I may have to replace it. The binding is just about to pop.