The simple definition would be, “the EPUB format is a free and open e-book standard by the International Digital Publishing Forum (IDPF)” (en.wikipedia.org/wiki/EPUB). EPUBs will reflow to fit any screen size; pagination and typography are controlled to a large extent by the viewing device.
An EPUB file is a ZIP file with its extension changed to “.epub.” You can extract the contents using any ZIP extraction utility. The contents of the EPUB consist of XHTML files and a CSS style sheet, an images folder, and a few other support files. While it is possible to embed fonts in an EPUB, some eBook readers will ignore them.
It is often necessary to extract an EPUB’s contents, edit them, and then recompress. Care must be taken to not break the file structure. For Mac computers, when you recompress your extracted EPUB contents you’ll need to use a script; for more information and a free script download see www.mobileread.com/forums/showthread.php?t=55681.
Your EPUB’s XHTML files have a limit of 300KB per file. Long documents must be broken down into smaller segments. EBook readers interpret each new XHTML document as a page break.
Almost all popular eBook reader devices currently support the EPUB; the one exception is the Kindle. Calibre is a free program that can convert EPUBs to MOBI format, which Kindle supports (www.calibre-ebook.com).
In order to view EPUBs on your computer
you’ll need to install a viewer. One option is Adobe Digital Editions (www.adobe.com/products/digitaleditions).
Invalid coding inside your EPUB can cause the EPUB to not work. You can test your EPUB in a free validator located at threepress.org/document/epub-validate.
I originally wrote this article as a handout for the March 2011 Portland IDUG meeting; you can download the handout in PDF format.