Good question, I'm a young fitness professional in that I've only been training for 6 months and learning for 4 years. I base alot of my buying on recommendations from people with more experience. Also at one time or another alot of the big names will put out a "reading list", the most comprehensive one being Eric Cressy's
http://www.ericcressey.com/recommendedresources.html
Also you are kind of doomed to reread some stuff, but then expand on that knowledge as long as it's a good text.
-Ned