Amazon's Kindle MatchBook service has gone live today, allowing customers to buy a heavily discounted Kindle copy of physical books they've purchased from the online retailer. Prices range between free and $2.99, offering a quick and cheap way to expand your Amazon ebook library. And on that note, there's good news: Amazon says that 70,000 books are enrolled in MatchBook at launch, dwarfing the original 10,000 titles it announced last month. More books will be added to the program every day, according to the company, and book detail pages will now list when specific titles will support MatchBook. Amazon combs through your entire order history going all the way back to 1995, so the initial list of ebooks offered to you may be longer than you'd expect. And since they're full-fledged Kindle copies, all of Amazon's signature features including Whispersync and X-Ray are included. To see which of your past purchases are eligible, head to Amazon now.