The CMS Medical Learning Network ("MLN") has posted an Evaluation and Management Services Guide for billing and coding. The Guide is a useful explanation of this complicated area and provides links to the 1995 and 1997 Documentation Guidelines for Evaluation and Management Services, as well as the Medicare Claims Processing Manual and the Current Procedure Terminology book. 

http://www.cms.hhs.gov/MLNProducts/downloads/eval_mgmt_serv_guide.pdf