What are Back of the Book Stamps?

Stamp Auction - United States Back of the Book Stamps and Postal History Carriers Stamps - The Erivan Collection of United and Confederate States Part V, lot 24

U.S. Back of the Book Stamps

Worldwide Stamps Archives - Jamestown Stamp Company, Inc.

Strange stamps lurk at the back of the book

Stamp Collector Magazine - Classic 19th-century stamps Back Issue

Airmails, postage due, back-of-book pages added to used singles album line

Stamps are Little Pieces of Art

Embossed Envelopes, Postcards & Back-of-the-Book Stamps.

H.R. Harmer GPN, Inc. Sale - 3031 Page 34

What are Back of the Book Stamps?