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Introduction to EPUB

EPUB (short for electronic publication) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .EPUB. EPUB is designed for reflowable content, meaning that the text display can be optimized for the particular display device used by the reader of the EPUB-formatted book. The format is meant to function as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale.


  • Free and open
  • Re-flowable (word wrap) and re-sizable text
  • Inline raster and vector images
  • Embedded metadata
  • DRM support
  • CSS styling
  • Support for alternative renditions in the same file
  • Use of out-of-line and inline XML islands to extend the functionality of EPUB

EPUB is essentially a ZIP format. If you change its extension ".EPUB" to ".zip", the EPUB file becomes a true ZIP file which can be unzipped. The unzipped EPUB files may be divided into three categories: XHTML documents, packaging files and container files.

  • XHTML Documents: XHTML is a strict version of HTML. The stricter syntax enables web browsers or eBook viewers to render the document more easily. This is important for reading eBooks on devices with limited computing power such as cell phones. In an EPUB book, the main content is placed in these XHTML documents, which can have .html, .xhtml or .xml extension. We prefer the .html extension because it can be viewed by most web browsers on various computing devices.
  • Packaging Files: They include two files: one with the .opf extension and another with the .ncx extension. The .opf file contains book information (author, publisher, etc.) and a list of all files in the book package. The .ncx file tells the sequence and organization (parts, chapters or sections) of XHTML documents in a book.
  • Container Files: They must be placed in a folder named "META-INF". These files are used for encryption and digital rights management (DRM). For an eBook without DRM, the container file has only a few standard lines.

Free EPUB Readers
There are various online or desktop tools as well as apps for mobile devices which allow users to read .EPUB formats, such as iBooks, Stanza, Calibre, Adobe Digital Editions, Bookworm, etc. Click here to learn the 7 featured EPUB eBook readers.

Create EPUB eBooks
A number of software tools are designed for making EPUB eBooks, such as Adobe's InDesign or Calibre or EPUB Builder. Also you can convert PDF to EPUB if you already have some PDF eBooks at hand.