What is an EPUB book format and what are its features?

Posted by Greten on 12 Sep 2020 under Terms

The EPUB is a file format used in electronic books or ebooks. EPUB means "electronic publication", and EPUB files end with a file extension .epub.

Unlike PDF ebooks, EPUB does not have a fixed layout. While it is generally advantageous, it can be unsightly for readers who want the appearance of books with professionally-made layouts typically found in PDF and printed books. The textbook publishing standard is to export the file from Adobe Illustrator as a PDF file and send the PDF file to a large printing facility. The publishing house can just sell the PDF file itself as an ebook. Hence, its appearance is closer to that of a physical book.

On the other hand, EPUB books are made with the expectation that it could appear differently across several devices. The layout can be a bit boring, but EPUBs feel more like real books in a different way; many reading applications render EPUBs as if they are real books with pages that you can flip sideways.

Features of an EPUB book

EPUBs are responsive. Similar to a responsive website, an EPUB book repositions its text, illustrations, and other elements so that it remains readable regardless of the device you are using. Images in an EPUB are typically aligned center with no text on both sides; they resize only if the viewing device is too small but otherwise, keep their original size. In large monitors, these illustrations will have large white space on the sides. In comparison, a PDF book has a fixed layout. If the PDF book's page is too large for the viewing screen, your only options are to either zoom it out, which may make the text too small to read, or view only certain parts of the page at a time, which is inconvenient.

A calculus textbook opened using the EPUBReader add-on for Firefox.

Another feature that can help make EPUB responsive is dynamic pagination. The page size of an epub depends on the size of the viewing screen; EPUB will put a certain amount of content that can fit in a page, put the remaining content to the next page, and then to the third page, and so on until all content are allocated to pages. Meaning, if you have a larger viewing screen such as a tablet, you will have fewer pages than when viewing the same book in a smartphone because more content can fit in the "pages" of the book as seen from the tablet. However, there is a minimum number of pages in EPUBs; in bigger viewing screen such as 13-inches monitor, you might see two or three distinct pages simultaneously visible. To keep seeing only one page, you may reduce the window size of the EPUB reader.

Since the pagination is dynamic, the table of contents in an EPUB book has no page numbers. Instead, each topic or header listed in the table of contents is a hyperlink that will allow you to jump to the page of the book where that topic fell in.

Technical details of an EPUB book

Similar to HTML5 and CSS3 specifications for webpages, and SCORM or Tin Cap API for elearning modules, EPUB also has its own standards maintained by a technical working group. The current version is EPUB 3.2, and it is maintained by the International Digital Publishing Forum (IDPF).

A collection of books, like shelves in a library, seen within the screen of a tablet computer.An EPUB is basically a very long webpage that, instead of scrolling downward continuously through the page, the reading application automatically puts the content into discrete pages based on the size of the viewing device.

During the ancient times—that is, during the time of ancient Egyptians and Babylonians—people are using scrolls to record, keep, and transmit a large amount of written information. Then, the Roman civilization invented the codex, from which our modern hardbound and paperback books evolved. The transition from webpage to EPUB is similar to that of from scroll to codex.

The EPUB file is essentially a ZIP file containing the aforementioned web page, the images and other media files that you can see on the EPUB book, and some other files needed to make it work as an ebook or be recognized by the ebook readers as an EPUB book.

Want to verify that EPUB is indeed a ZIP file? Make a copy of an EPUB file and put it in a folder different from that of the original file. Next, rename and change the .epub extension to .zip. After renaming, right-click on it and select "Extract all". You will see in the extracted folder the files that made-up the EPUB ebook.

Applications to open an EPUB book

The interface of Google Play on a tablet, showing the different applications that matched the search query: epub readerSeveral applications allow you to open and read EPUB books. If you are using an Android device, such as a tablet computer or a smartphone, all you need to do is open Google Play and search "epub reader". There are several applications to choose from, but I would like to recommend the following:

  • ReadEra, by ReadEra LLC
  • Lithium, by FaultException

These two applications index all the EPUB and PDF files in your mobile device's storage drive and display the book covers as icons that you can tap to open the book. These two have the following advantages:

  • Among the top-rated EPUB readers in Google Playstore
  • Require only access to file folders and nothing else
  • Show no ads
  • Free

I am not sure if other EPUB readers have unique features and functions, but I haven't tried yet. For my usual reading purpose, these two applications are sufficient.

For computers, I recommend Calibre as a standalone application. It is a free and open-source software (FOSS) application available in Windows, MacOs, and Linux. You can download the installer from Calibre website.

Firefox and Chrome also have add-ons or extensions, making these browsers capable of opening EPUB files. In Firefox, the most popular (in terms of the number of usages and the score in ratings) seems to be the EPUBReader; it even includes a "Recommended" marker. In the Chrome Web Store, EPUBReader is also available as well as the similarly-named EPUB READER; both are from the same company. However, the most popular in Chrome is Epubx.

Of course, there are other extensions and add-ons for EPUB reading that might be suitable for your need even if they are not highly-rated or used by fewer people; try them if you want but proceed with caution.

Where to get EPUB books?

You can buy EPUB books from some of the major publishing companies and online bookstores, including, but not limited to:

Last updated on 12 Sep 2020.

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