I hadn't checked on this thread in a while and it seems more have posted since I last looked. From what I gather, it is relatively easy to find what is in any particular book, except perhaps the very obscure books. However, knowing who might own a book might be useful.

FWIW, I often use worldcat.org to find the contents of a song book. It has limited usefulness to search for a song title to find a book that might contain it. Although refining my search techniques might improve my results.