After the announcment from Apple and its new iBooks 2 for iOS and the new iBooks Author for Mac, I just wanna give you a sneak peek inside the new textbook format and compare it to the ePub standard.

Comparison of iBooks VS ePub

1. File extension


First of all what you get by exporting your freshly written textbook as iBooks, is a file with the ibooks extension.


The normal file extension is epub.

2. Mimetype


The textbook format has a mimetype of application/x-ibooks+zip


Here we have application/epub+zip

3. Directory structure


Describing the contents of the book with all its files, Apple uses the OPS directory and inside the normal *.opf file outlining the structure of this book. Inside this folder there is also an extra container called assets. This is where all the “magic” is happening. You can find CSS, Javascript, SVG.

  • assets
    • CSS
    • hints
    • images
    • ncxExtensions.xml
    • search
    • svg
    • thumbs

Only looking inside the ncxEntensions.xml file unveils a normal Apple Properties PLIST. That means, no ePub 3 standard.

Look inside the search folder. There are files used for fast searching, not only for the written words, but also to the stem of the word. All precalculated on the powerful Mac hardware, instead the iPad itself.

The META-INF directory is the same like in ePub.


There are no SVG or search folders.


As we can see, Apple extended the ePub format and by changing not only the file extension but also the mime type, the resulting textbook is only readable by iBooks. You can try to change the ibooks extension to epub and sometime your ePub reader will open that file, but the result is not good looking.

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