Format Guide: EPUB | MOBI

EPUB and MOBI are the most common formats for reading eBooks on eReader devices and tablets. Apple’s iBooks, Barnes & Noble’s Nook, Adobe Digital Editions, and Aldiko on Android all support EPUB. Amazon’s Kindle uses MOBI, a software package created by the French company Mobipocket, which was bought by Amazon in 2005. MOBI eBook files usually have the .prc or .mobi extension.

The advantage of EPUB and MOBI is that both formats display reflowable content (the text adjusts to fit the size of the screen on your reading device) as well as fixed-width layout. Both formats are also open standard, which means that they are free and publicly available, although rules exist regarding their usage.

EPUB and MOBI files can be edited using a number of eBook editing tools. MOBI should first be converted to EPUB, after which the file can then be edited using one of the tools described in this guide. A variety of MOBI to EPUB conversion tools are freely available online. lists many of these tools with crowdsourced reviews.

Editing an EPUB

An EPUB is essentially a ZIP file containing nested folders with XHTML text, images, Cascading Style Sheets (CSS), embedded fonts, and metadata files. Almost anyone can edit EPUB files with the help of an eBook editing tool, although some familiarity with hypertext markup language (HTML) is helpful.

Original Format

Possible Editing Tool (Web)

Possible Editing Tools (Desktop)

Possible Output

MOBI Convert to EPUB to edit
EPUB Pressbooks



Pressbooks: PDF, EPUB, HTML, .wxr

Sigil: EPUB

calibre: EPUB

Sigil and calibre are open source software applications that can be downloaded freely from the Web. Both are maintained by volunteer developers, which means that user support is limited. The user instructions provided below are very basic. The best source for help is usually one of the online forums where users exchange information and answer questions.


Sigil can ingest .txt, .html, and .epub. The output type is always .epub. Using Sigil requires some knowledge of editing HTML. CNET offers technical documentation and free software downloads for Sigil.

For additional help, see MobileRead’s Sigil Forum.

Additional Sigil resources:


  1. Install calibre on your computer.
  2. Download the EPUB version of the open textbook you’d like to edit.
  3. Open the textbook in calibre.
  4. Click on “Add books” to add your book.
  5. Click on “Edit book” to launch the editor.
  6. Double click the section/chapter of the text you want to edit.
  7. Add/delete/modify the text. A preview of your edited text will appear on the right side of the window.
  8. To add an image, click on the “Insert image” button. Note: If your image is very large, you may have to play around with the HTML to adjust the size.
  9. Don’t forget to save a copy as you go along!
  10. To change the cover of your book, go back to your calibre library. Right-click on the book. Then choose “Edit metadata” and “Edit metadata individually.” Edit the new title and author, and then click “Generate Cover.” (Make sure to click “OK” so you know it’s saved.)

calibre offers a Help section on its website. Kovid Goyal has also written a guide to editing eBooks in calibre.


Pressbooks can ingest EPUB and output into read-online-HTML or downloadable EPUB, MOBI, or PDF. If your institution is a member of the Open Textbook Network, contact the OTN for information about access to a shared instance of Pressbooks. Or, you can download and host your own instance.

Start with Step 3 of this Pressbooks guide to import your EPUB into Pressbooks. (This step also covers ingesting WordPress files into Pressbooks.)

For more information about editing in Pressbooks, see the Pressbooks User Guide.

Contributors: Annie Johnson, Anita Walz, Peter Potter