Project Gutenberg is essentially an archive of over 33,000 free eBooks that users can load onto most eBook readers. Magic Catalog, from Project Gutenberg, is a free program that acts as a middleman between eBooks and eBook readers, specifically Kindle and iBooks. The app contains links to the free eBooks offered by Project Gutenberg, which when selected, will load said free eBook into one’s Kindle or iBooks.
There is also a more roundabout way available to load eBooks into one’s reader using the program. From the Unofficial Apple Weblog:
If, for some reason, you’d prefer to download the books to your computer and transfer them to your iOS device (or if you already have .epub, .mobi, or other e-books on your computer), you can do so using iTunes. If you have the Kindle app installed, it will appear under the “File Sharing” section in the “apps” tab, and you can add books there.
You may notice that iBooks does not appear in the “File Sharing” section. To transfer books to iBooks, simply drag to the “Library” section of your iTunes library (see this page at Apple.com for more details).
Kindle and iBooks: When you are on the web page to download the book, you will not see the name of the book; you’ll see something like “pg23.mobi” followed by the size and a button to open the book in the Kindle app (or iBooks if you have the EPUB version). Once you open the book in the appropriate application, it will show the correct name.
Though Magic Catalog seems like a quick and convenient way to load free eBooks onto one’s reader, a few users claim that browsing through the many free eBooks is tedious, as Magic Catalog doesn’t quite have an organizational system in place as of yet.