Schweres Panzerjäger-Regiment 656 is one of the better researched German formations of World War II, and there are several really good books on the regiment available, although some of them are fairly expensive at this point. There are two books by Karlheinz Münch available which cover the Ferdinands: Combat History of Schwere Panzerjäger Abteilung 653: Formerly the Sturmgeschütz Abteilung 197, 1940-1943 and Combat History of the 654th Schwere Panzerjager Abteilung. I consider both of these to be "must haves" for any dyed in the wool Ferdinand fanatic out there.
If you're interested in fielding the regiment in Flames of War - stay tuned for the next article in the series on the Flames of War main page. You can also look at some of your modeling options in a previous blog entry, as well as a feature on the new battlefront miniature. Happy gaming!