What is the BACKSTRIP in relation to books?

"Backstrip" is not a standard term used in the context of books. It's possible you might be thinking of one of the following:

* Spine: This is the vertical edge of a book that holds the pages together and displays the title and author information.

* Back cover: This is the back side of a book, often featuring a summary, author bio, or other relevant information.

If you can provide more context or a specific sentence where you encountered the term "backstrip," I can help you understand its meaning better.