We list every edition of cookbooks - hardcover, paperback, eBook, special anniversary edition, etc. Each of those editions has a different ISBN number, the unique identifier of an edition. Many members like to add the edition they own to their Bookshelf. To do that you would open Import Books, accessed from the My Bookshelf tab. Type the ISBN number and click Add Books. If the book is already in the Library it gets added to your Bookshelf immediately. If it is not yet in the Library, we try to import the book data from Amazon and the book then gets added to your Bookshelf. If Amazon doesn't list that book or they neglected to add a food or drink subject to the book, then you would need to add the book data manually, using Member Added Books.
The reason to use Import Books rather than searching the Library by ISBN is that linked books are "hidden". Books get linked when one edition is indexed. Unindexed books do not get linked since the reason to link them is to share the recipe index, recipe notes and rating, and recipe photos, across all editions. You can view all editions of a book by opening an author page (click on the author name). All their editions are listed, in alpha order, so all editions of the same book will be grouped together in the list.
When you are seeing one edition on 50 Bookshelves, and another edition on one, that is usually the print edition vs the eBook edition.
Re your question about books on high rotation, you do need to add a bookmark if you want to limit your recipe search results to favorite books.