A Guide to Folktales in the English Language: Based on the Aarne-Thompson Classification System (Bibliographies and Indexes in World Literature) by D. L. Ashliman was another early and important discovery when I was first starting SurLaLune. For full disclosure, I found this in the University of Tennessee library reference section and desperately wanted to take it home. I only "needed" the Bluebeard information at the time, but I was fascinated by all of the other tales living within it. I was vaguely familiar with the Aarne Thompson classfication and had found the motif indexes which were almost incomprehensible to me at the time. I cut my classification teeth on Ashliman's work which also inspired his own excellent Folktexts website which started a few years before SurLaLune but I didn't discover it until after I had launched my site. Search engines weren't nearly as efficient then despite the considerably less volume they sorted through in those days. (Do I sound sufficiently old-fogeyish?)
This book, alas somewhat dated now since the ATU system is now receiving preference in most circles, is still very useful. What it does is gather lists of English language tales under their classification number, a simplified ATU system manual similar to Ashliman's website, but without the full text tales themselves. Of course, all the books included were published before 1987 when the book was published, so the last 20+ years are not included, but this makes an excellent and easy start to hunting down variants of tales available in English. Ashliman's site is limited to out-of-copyright tales as well his own copyrighted translations, but the book covers hundreds of fairy tale anthologies that are still in copyright and can often be found in university libraries. Ashliman's site builds on the most popular tales as does my own SurLaLune site for the 49 tales annotated there, but a significant amount of unique information is available in the book, especially for less popular tale.
If that is rather confusing, here's a basic description from the publisher:
A Guide to Folktales in the English Language is designed to assist both the folklore specialist and the general reader in locating the texts of folktales published in collections. Author D.L.Ashliman follows the widely accepted type classification established by folklorists Annti Aarne and Stith Thompson and last revised in 1961, organizing more than 5,000 stories and episodes under some 1,000 basic plots. Each plot is presented in capsule form, followed by the titles and essential bibliographic data of published variants. The result is a comprehensive overview of all major European folktales that will be invaluable for students of folklore, literature, and popular culture.
Each classification begins with a basic summary of the tale type, too, often based upon its most popular variant. It's great reading if you like this kind of stuff. I do obviously, so this book is a library essential. At times it has lived on my desk and avoided shelving for weeks at a time. When I am actively working on a fairy tale project, it is one of the first books I consult. And when it is shelved, it's only because my desk is being cleared or I need to have it where I know I can find it again easily. It's not inexpensive, but it has earned its cost back over and over again.