While I am not a big collector of Castle Films, they certainly have been a very important part of the home film industry and probably is how many people started out collecting films years ago. I have found the reference book, "Castle Films: A Hobbyist's Guide" by Scott MacGillivray to be very helpful.
This book is over 400 pages and contains a very interesting history on Castle Films from the early years up until the Universal 8 era began in 1977. Most of the book is indexes of all of the various films by subject--news, sports, cartoons etc. Other indexes by serial number or alphabetical by title or character make it pretty easy to find a title. Many of the films have additional details which help decipher title differences that seem to occur frequently between a digest and the original feature film. Castle did issue some real gems and when you read through the lists in this book it is quite incredible the sheer number of films they put out over the years!
I think this is a great reference book to have, and it is still available through Amazon I believe.