A few off the top of my head that may be worth checking out...
- Amtrak trains and travel by Pat Dorin
- All aboard Amtrak 1971-1991 by Mike Schafer
- Amtrak by the numbers by Elbert Simon and David C Warner (1971-2011 roster info)
- Amtrak in the Heartland by Craig Sanders
- Amtrak Heritage 1971-1977 by John Taibbi
- Amtrak by Brian Solomon
- Amtrak Across America by John A. Fostik
- Amtrak Car Spotter A Guide to the New Amtrak Car Number by WD Edison
- Morning Sun books has several Volumes on Amtrak
- Passenger Train Journal and Passenger Train Annual (various issues)
- Trains Magazine (Various issues)
- Pat Dorin also published many books on specific railroads passenger trains that covered Amtrak as well, Atsf Milw etc...
