I just got “Master Your Mac: Simple Ways to Tweak, Customize, and Secure OS X” yesterday and am amazed with the amount of material covered in it.
It’s almost 400 pages of great material that is useful for everyone from newbies to veterans. It contains simple shortcuts, using the new features of Mountain Lion, using the hardware, configuring the software, and even shows useful third party applications (many of which are free).
My favorite section so far is simply a Bluetooth Proximity Monitor. This chapter tells you how to setup your mac so that if you walk away from it with a connected bluetooth device, it can automatically run whatever tasks you want. For example, pause iTunes, set your away status in your chat app, and even lock the screen. And then it can do the opposite when you get back with the Bluetooth device.
With 38 chapters of information, there will be something for everyone. I highly recommend this book if you have a Mac.