What Are ePub and Mobi Files?

You’ve heard the terms. You know they’re electronic books. But what is an epub or mobi file?

Officially the term “epub” is short for “electronic publication” and is a free and open ebook format used by the International Digital Publishing Forum (IDPF). It is able to be read by most ereaders, including all Apple products, Android devices, Nook, Kobo, Sony and Windows. The only devices that currently don’t read epub files are Amazon Kindle devices.

The “mobi” format was created by a small company for the Mobipocket reader, but was purchased by Amazon. It is nearly identical to the native azw format that Amazon uses, and is used almost exclusively by the Kindle, although some readers have apps that can be installed to read them.

What does that mean for a writer? Or even a reader (both human and digital)? Well let’s explain a little more about what makes up an epub or mobi file as opposed to, say, a text file.

What’s the point of a digital format?

First, these files were created with flexibility in mind. They have the ability to incorporate code much the same way that a website would, and therefore allow the book to be re-sized, hold embedded images and video, contain sound bits, and even make use of security measures to avoid pirating. A simple text file can only put out exactly what you put into it, but an ebook format can turn reading into a truly interactive experience.

It’s the reader (the digital reader) that forms the book. The reader creates the ideal format for its viewers, using the information provided in the files to provide the perfect reading experience. Text files are static things, performing in their limited scope and sometimes failing miserably and distracting the viewer from the content within.

The two ebook formats, however, are fundamentally different.

The epub format can be thought of like a container, very similar to a zip file. Within the epub are all the resources that are needed to make up an ebook, from the text and images, to the code required. A reader will then access the container much like the internet accesses the code of a website and will put it all together for viewing.

The mobi format is a single file, created by mushing all the ebook’s content together with a few instructions for its reassembly. A reader can take this file and use the included instructions to recreate the ebook for the viewer. (Note: as of this posting, the mobi format doesn’t support audio/video.)

Because of this difference, most readers can only process one or the other of the files, not both. There are file converters available on the internet which will attempt to convert one to the other, but sometimes data or styling is lost in the process.

How does Simply Written do it?

Simply Written treats these files separately, preventing this corruption. We work with the raw data an author enters into the site and custom package the files either as epub or mobi. The files created in this manner are ready for viewing on a reader, or for publishing on another website such as Amazon or the iBookstore.

To find out more about how Simply Written converts files, read About Simply Written.