Marxtin.com has some info, and, I think, used to offer a DVD as well.
The Greenberg books are often available on eBay, but run $80 or more usually:
"Greenberg's Guide to Marx Trains", c. 1985 - By Eric J. Matzke - Inclusive and fascinating.
"Greenberg's Guide to Marx Trains Volume 1", c. 1989 - Eric J. Matzke - Expanded so that 2 volumes were necessary. Predominantly tin and steamers. Lots of history.
"Greenberg's Guide to Marx Trains Volume 2" - c. 1990 - Eric J. Matzke. If you are a fan of the large plastic E7 diesels, they are shown here in all their full-color glory. Thirty-seven pages on plastic freight cars. The big omission is no steam engines.
The last two have lots of great color photos of the equipment.