Most people know that there is a free Kindle app for iOS that 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 taps, you can have the new book on your iOS device without ever needing to go through iTunes and without spending a penny.
The easy way
If you already have the iBooks or Kindle apps installed, simply go to Magic Catalog on your iOS device and tap the appropriate link for whichever app you are using (MOBI Edition for Kindle, EPUB Edition for iBooks).