Books
I've found the following books to be interesting and useful. Check them out for yourself on Amazon.

American Flyer Pocket Price Guide 1946-2010 (Greenberg's Guides)
If you are going to buy and sell American Flyer trains, you need an accurate price guide to make sure you don't pay too much or charge too little. This pocket guide by Kalmbach Publishing Company is the most current price guide available for American Flyer S Scale trains. Published in October 2009, it gives accurate, current values for American Flyer S Scale trains and accessories manufactured by A.C. Gilbert and Lionel between 1946 and early 2010. At only $10.85 it's very affordable, and it's compact size makes it easy to take with you to train shows and swap meets. If you want to get into American Flyer S Scale trains, this book is a must... unless of course you don't mind paying too much. Seriously, this book could easily pay for itself in short order. If you want to know what a particular American Flyer locomotive, car, or accessory is worth before you spend your hard earned money, you need a book like this. In addition to pricing information on American Flyer trains, this book also features information on currently available, ready-to-run S Scale trains information from manufacturers such as American Models and S-Helper Service.

Standard Catalog of American Flyer Trains
Published in 2007, the pricing information in this book isn't as current as the American Flyer Pocket Price Guide,
but the full color pictures make it easy to accurately identify American Flyer trains. Unlike some books, most of the photos are bright and clear. Although it is primarily focused on the S Scale American Flyer trains, it also contains information on American Flyer wide gauge and prewar HO and O gauge trains. It's probably the most comprehensive overview of the American Flyer line available - at least for their S Scale line.