Most books you read already are likely to sit on your shelf without and further use.
An app that lets you take a picture of a book (or the barcode) recognizes the book and shows you where you can buy it cheaply as an ebook. You can immediately purchase the ebook, and donate the physical copy on your shelf. You win by freeing space for new books, especially the kind that is much better in print (special editions, art albums, etc.), the people you gift the paper copy to win, and the authors and publishers win.