Working on a recipe and trying to pick out what hops you want to use or do you have a recipe with a hard to find hop and need a substitution?  The most readily available (free), current, and easy to use compilation of hop information can be found at the Hop Union website: http://www.hopunion.com/hopunion-variety-databook.pdf 

You can pretty much find all of the pertinent information on the major hops available in that handbook including information on alpha acids, beta acids, co-humulones, oil components, uses, and other background wisdom.

For formulating your hop breakdown in recipes, there are few books better than "Designing Great Beers" by Ray Daniels.  Also, the BJCP style guidelines can provide some wisdom as to the families of hops to use for a certain style.  http://www.bjcp.org/stylecenter.html