It's pretty tough to beat this pocket reference to emacs. It makes looking up both common options and less common commands and modes simple and quick. If you're an emacs user, this book is a must have -it's easier than apropos and quicker than C-h ?.
If you are new to Emacs, the I would recommend this book to you. It gives you all of the key commands that you need right at your fingertips. If you new to learn Emacs, get a full featured book, but also get this one. You will probably find yourself referencing this book quite often until you remember all of the common keystrokes.
Great book for beginners and those of us who haven't used emacs in awhile and need a little reminder here 'n there. There are many more extensive books on emacs, but this is great for a handy reference without the bulk of a comprehensive book.