These books are very helpful if you have a piece of a bottle with distinctive marking, like your seafoam piece shown above.
They concentrate on collector's bottles but should have the bottle that your piece came from. We would recommend Michael Polak's book, Warman's Bottles Field Guide
Known as the "Bottle King," Michael has a collection of over 3,000 bottles acquired over 30+ years. A well-established expert on identifying bottles, he has written six editions of the Antique Trader Bottles Identification and Price Guide and two editions of Antique Trader Bottle Field Guide.
A nice thing about this book is that it is packed with actual photos and shows clearly
the details of the bottom lettering on many, many bottles.
So there's a good chance you could identify what you've found. It could be quite rare.
If it is a piece from a wine bottle for example from Italy, it will be harder to identify, as few books are written specifically about wine bottles. But this book is fairly inexpensive and definitely should be in every sea glass collector's library.