GNU Emacs Pocket Reference

As Emacs is my favorite editor, this hand book comes in really handy, especially if you don't use some of the items enough to need refreshing. I don't always use the dired mode, but find it useful in a number of areas, so it's always at my fingertips.

Yes, yes, emacs has free help available online, help built into the program, and so on. That said, sometimes a quick reference is helpful when you're first learning.

I love O'Reilly Ppcket Books. Once one masters a subject pocket books provide an ideal reference and memory jogger.