Flower Frog Resources
As a new collector, I sat in the reference section of my county's main library looking through dozens of books on pottery and glass. It was disheartening to find that most made no mention of flower frogs.
The following books address flower frogs enough to make them worth a trip to your library. And if your library doesn't have a book, I suspect they can borrow it for you through an interlibrary loan.
Published in 2001 and currently out of print, this excellent book is usually available on eBay.