There are dozens of e-book formats that you can access on your iPad, but these are the most common ones:
Apple Books
The books purchased through the Books Store are in the ePub format, but they are modified to include Digital Rights Management to prevent unauthorized sharing or copying. Any ePub digital book can be opened in the Books app or in other downloadable apps.
ePub
The ePub open format is one of the most commonly used e-book file types. Apps like iBooks and Nook can read ePub files purchased from their respective online stores or downloaded from the web. Several programs for Mac and Windows convert other kinds of e-books to ePub format.
Barnes & Noble Nook
Barnes & Noble sells e-books on its website and through its Nook app. Access these books using the free Nook app available at the iPad App Store. Nook e-books are a renamed version of the common ePub file type.
Kindle
PDF is probably the most popular downloadable document format on the web, so you’re sure to find e-books in this format in a lot of places. There are tons of PDF-compatible apps for the iPad, including Adobe Acrobat Reader, GoodReader, and iBooks.
CBR and CBZ
CBR and CBZ formats are related types of digital book formats that are used to present comic books and graphic novels. To read them on the iPad, you need an app such as the free Manga Storm CBR app or the paid app Comic Zeal.
comiXology
KF8
Kindle Format 8 is the next-generation version of the Kindle e-book file. It adds support for HTML and CSS to the existing Kindle format and uses the .AZW3 extension. The Kindle app on the iPad supports the KF8 format.
Microsoft Word
Microsoft Word creates .docx files. Some e-books, usually those sold as direct downloads by self-publishers, come in this format. While there are a number of iPad apps that can read DOCX files, Microsoft Word for iPad and Office for iPad are both free.
Mobi
Plain Text
Plain text files, which have the .TXT file extension, pop up from time to time, especially on sites that offer free, public domain books, such as Project Gutenberg. A large number of apps support plain text files, including the paid app GoodReader, and iBooks.
iPad Audiobooks Support
The iPad can also be of use if you prefer to get your books in audio form rather than text. Some of the most common audiobook types supported by the iPad include:
Audible: Audible.com uses its proprietary formats (AAaa, AAX, and AAX+) and is probably the most popular and widely used audiobook service on the internet. With the free Audible.com app for the iPad, enjoy any book from that site on your tablet. MP3: Other audiobooks are often delivered as standard MP3 audio files. Because that’s the same format as many music files, download an MP3 and then sync it to your iPad to listen to it using the built-in Music app.
