I should have added that the book 'Modeler's Guide to Realistic Painting & Finishing' also includes information on how to paint with brushes and spray cans, and might be more useful for people just starting out. The first edition of it is good, even if it is a little dated (in terms of the airbrushes, compressors, and paints that it covers, but you can find it online as a PDF somewhere), but still very useful, and a completely updated version was published more recently.
http://www.kalmbachstore.com/12257.html