Free books. Does anything on earth rival free reads? Perhaps free beer. Maybe free pizza. But that's it, as far as I'm concerned. It's been a slippery slope. Way back in 2005, Amazon.com announced a new program called "Prime." It was a simple proposition: pay $79 a year for the annual membership, and you'd get free two-day shipping on most of my orders. Now, several years later, Prime comes with a bunch of free content, including books.Of course, that's merely one source of free books. One of the best things about Amazon's digital bookstore is that many of its most popular books are offered completely free of charge during brief promotional periods. If you manage to find and download a book while it's offered free, it's yours to keep-forever. It's a great way to sample a new genre, or perhaps discover an author you hadn't noticed before.Hundreds of websites have popped up across the Internet to highlight free and discount Kindle books.Sometimes it seems like we're being overrun by free content. But really, just some of it is worth bothering about. This book will help you figure out what's worth your time.