It is a moot point whether book or dust jackets count as ephemera. When taken from the book, perhaps. We have a collection of Jackets in the John Johnson Collection, which are catalogued but not digitised. The Bodeian's jackets are also sent to us to process, but that is another story.
The New York Public Library has 2,511 jackets online, from American and European books.
(c) New York Public Library
And, while on the subject, G. Thomas Tanselle's long-awaited book, Book-jackets: their history, form, and use, has just been published by Oak Knoll (available in December).
(c) Oak Knoll
Librarian of the John Johnson Collection