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Perhaps you've heard the term EPUB and thought oh yeah, that's an eBook. Well, yes it is one form of an eBook, but the EPUB format is actually a compressed folder of many different files. EPUBs can be opened on most any device with an EPUB reader application installed. The application interprets, then displays the compressed files one at a time in a linear fashion, though you can navigate between them with an interactive table of contents. The content of an EPUB is re-flowable, which makes it easy to read on any size screen EPUB supports some basic interactivity, images and even movie and audio content.
Typically EPUBs are used for text heavy documents like novels. Each chapter or section of an EPUB is represented by a single HTML file, that's right, the very same coding language used by most websites. So is an EPUB a website? Well, they can be considered more of a "micro site," rather than a website. You see, EPUBs lacks many of the features of a traditional website, but if you were to decompress an EPUB file, you would see a folder structure similar to that of a website.
You can even create or edit an EPUB with web development software like Adobe's Dreamweaver. Let's take a closer look at an eBook document to see what's going on behind the curtains. So here I have an EPUB, it looks and behaves like any other single file. If I double-click, it opens in the appropriate software to display the file. To the untrained eye it would pass as an ordinary document, but there is more here if you know where to look. I'm going to decompress this EPUB.
I'm left with a folder. So if I open this folder, I have two more folders and a file. The file is called mimetype. This file tells the devise that this is an EPUB. If we look in the META-INF folder, we'll find two XML documents. These files describe the contents of the EPUB. Opening the OEBPS folder reveals a list of xhtml pages. These pages are the pages of the document or the sections of the document.
There's a font's folder that contains the fonts that were used in this document, and there's an image folder that shows all the images that were used in this document. There is even a css folder, opening it reveals the css page. The CSS or Cascading Style Sheet controls the display of these xhtml pages. The code describes things like fonts and color and positioning of text and images contained within those pages.
There are two more files located here. The first is the content-opf; this file tells the EPUB reader what is inside the EPUB document. You'll also find the toc.ncx file. This is the interactive table of contents readers use to quickly navigate your EPUB. There are EPUBs that don't act like EPUBs; instead they seem to be empowered with features reserved for other forms of digital publications. Features like movies and audio tracks, embedded scripts, and even object positioning.
Historically, EPUBs with features like these have been described as Enhanced EPUBs because they were more than your average fare. Many of these enhanced features are now included in the EPUB 3 Standard, though you may still hear it occasionally the term Enhanced EPUB has fallen to the wayside and been replaced by EPUB 3. For the latest information on the EPUB 3 Standard visit the idpf.org website. There is more to an EPUB that meets the eye; a lot goes on behind the scenes to make even the simplest of EPUB's function the way we have come to expect.
Peeking out of the hood of an EPUB reveals its website like construction with HTML and CSS files abound. Knowing, what's at the core of an EPUB will hopefully give you a better understanding of the features and limitations this digital publishing format has to offer. If you intend to publish EPUB documents on a regular basis, I strongly suggest you learn a little more about the underlying code languages, HTML and CSS. You may never have the time a single line of code, but it would not hurt you to understand these core components of the EPUB format.
For more information, about HTML and CSS, I recommend James Williamson's CSS Fundamentals and Bill Weinman's, XHTML and HTML Essential Training here in the lynda.com Library.
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