1st. Main Reference: The Encyclopedia of Morgan and Peace Dollars 4th edition by Leroy C. Van Allen and A. George Mallis.
2nd. Reference: Updates to the Encyclopedia of Morgan and Peace Dollars - The New Vam Varieties of Morgan and Peace Dollars 1992-2006, which you can get from Leroy C. Van Allen.
3rd. Reference: The Top 100 Vam Keys by Jeff Oxman and Micheal S. Fey Phd.
4th On Line Reference: Vam World at http://www.vamworld.com/ I highly recommend all my customer to join this web site for great vam infomation.
5th Peace Dollar Reference: SSDC Top 50 Peace Dollar Varieties by Jeff Oxman and Dr David Close
6th Software Reference: There is software available for the top 100 Vam's, 1878 series of coins finding treasures in 1921 Morgan just to name a few good ones.
7th Cherrypickers Guide: Fourth Edition Volume II Half Dime to Modern Dollars by Bill Fivaz and J. T. Stanton