To add a new selection to an excisting selection, just press Shift while drawing the second selection (or press Alt to remove the new selection from the old selection).
Which book to buy depends a bit on what you want to do with photoshop. If you are working mainly with photo’s, I can advice Scott Kelby’s “photoshop CS5 book for digital photographers”as an excelent starting point.
Graffity works as a plugin. So you can prepare your project in the keyframer, but you will always need a host to render the project. For stand-alone rendering, you will need to upgrade to RED.