Why are these books free? It's because they are in the "public domain," which means they were published before 1923 or for some other reason are not subject to copyright restrictions. There are literally millions of books, and even films and music, that are in the public domain. A great place to learn more about public domain materials is an online project called the Public Domain Review. If you are interested in searching for public domain books, a great place to look first is at the Penn Online Books project.
There are a total of around 200 free Kindle eBooks (browse the complete list here) that pop up in the widget at random, and all the books are related in some way to mythology and folklore or to the Indian epic tradition. No matter which class you are enrolled in, I hope you will find some of the books intriguing enough to take a look. Each of those books has a link to the Amazon page where you can download the book, and there's also a "more information" link that will take you to a blog post where you can learn more about the book, including a table of contents when available. During the semester, I'll be featuring one of these free eBooks every day based on that week's readings for class.