Category Archives: Tech Tips

Tech Tips: What is an ePub?

In the coming weeks, I’ll be contributing a series of entries discussing how to set-up your InDesign files to create ePubs that require minimal interaction with the xhtml code. I will also show you how following those steps in InDesign will translate to your ePub file. But, first, I think it’s important to fully understand what an ePub actually is.

ePub is the standard e-book format established by the International Digital Publishing Forum (IDPF). ePubs are files (with the extension .epub) that display reflowable content across many different eReaders (Apple iBooks, Nook, Kobo, etc.). The reflowable content can be optimized to the display of the device or to meet the reader’s needs.

An ePub is essentially a collection of files zipped into a single file. You can see all of the inner workings of an ePub by simply changing the file extension from .epub to .zip and unzipping the file. The preferred method would be to view the contents through an XML editor. I use Oxygen XML editor, but a good and free ePub editing tool is Sigil. These programs allow you to open and edit an ePub without unzipping it. (Unzipping an ePub can lead to some issues because you’re required to re-zipped in a particular order. This has to be done manually through command-line prompts.)

An ePub file contains the following files and folders:

    Mimetype: identifies the package as an ePub
    META-INF folder: usually only contains the container.xml file
    Container.xml: points to the file defining the contents of the book (opf file)
    OEBPS folder: location of the content of the book
    images folder: contains the ePub’s images
    html/xhtml files: these are the text files of the book
    .css file (cascading style sheets): defines the formatting of the text and other layout attributes
    .opf file: XML file that list all of the files that are contained in the ePub and specifies the order in which the html/xhtml files should appear. This file also contains the book’s metadata
    toc.ncx: XML file containing the navigational table of contents


The above ePub structure is the ePub version 2.0.1 structure. There are some changes with the newly developed ePub 3.0 specifiations which expands ePub to include html5 elements such as audio and video. For now, I will focus on ePub 2.0.1.

In my next post, I will focus on some general good practices to follow when setting up your InDesign files.

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