If you've got some time on your hands and enjoy reading, one of the better investments I made as a photographer was to pick up a copy of Ansel Adams's Technical Series. "The Camera", "The Negative", and "The Print" are well worth the read. Some of the material may seem a little dated (who's printing on Seagull paper?) but in fact most of it translates quite well into digital photography. I'm re-reading "The Camera" right now, and it's an outstanding refresher.