I'm going to bump one of my old threads -- in the initial post, I mentioned a small book published by PSE: Professional Stamps Experts. Several of us were able to request a free copy of the book. I don't think that offer is still valid.
However, while surfing, I found they have posted a PDF version of the book. It makes a nice supplement or substitute for the printed version.
http://www.psestamp.com/usd/moreinfo.chtmlKirkS