Or the eBook.
Amazon used to have a service called "Amazon Upgrade" where you could pay an extra $5-20 for a book (that might have cost $20-100) and also get access to a web-hosted version of the book. I did that with several of my math textbooks in grad school. When the Kindle came out, they deprecated Amazon Upgrade, and now it's very difficult to find the ebooks I already paid for (though they are still available on the site).