Most people know that there is a free Kindle app for iOS which will let you read books from Amazon's Kindle store. Apple also offers a free iBooks app. Fewer people have heard of Project Gutenberg, where you can download over 30,000 free books, and fewer still have heard of the Magic Catalog from FreeKindleBooks.org.
The "Magic Catalog" isn't where Harry Potter shops, it's a "book" containing nothing except links to other books. You can use the search feature of your favorite e-book reader to look for books; with a few