Announced back in September, a new service from Amazon that more closely ties together printed books and e-books recently made its debut. But what is Amazon MatchBook and how does it work?

While electronic books have become increasingly popular, publishers have not been quick to embrace bundling a digital version when you buy a printed book. Amazon's service offers a Kindle e-book version of select books at a steep discount, from free to $2.99, depending on the title.

Even better, MatchBook works for books you've already purchased in the past from Amazon; it can search your shopping history and tell you which books are eligible, dating all the way back to purchases made in 1995.

Unfortunately, at the moment, the MatchBook selection is quite small so not all books are eligible for the Kindle discount. Amazon says about 70,000 books are part of the program now, but that number is likely to grow over time if more publishers jump on board.