The EPUB format is a standard eBook format recommended by The International Digital Publishing Forum. It 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.

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.

Currently, the EPUB format can be viewed by Calibre, Adobe Digital Editions (ADE), FBReader and Stanza. In addition to notebooks and desktops, ADE supports Sony Readers, Stanza targets Apple's iPhone, and FBReader applies to Google's Android.

Calibre renders CSS and table of contents very well, but it cannot highlight or annotate.

ADE has the highlighting and annotation features. However, scrolling is not smooth when there are images or tables. Another problem is that the table cannot be centered, which is important for displaying poems (see Mike Cane's blog).

Both ADE and Calibre do not run on any smartphone yet. ADE requires Flash Player which will be available on most smartphones next year (News). Perhaps we can see ADE on smartphones in 2010.

FBReader is still in the early stage of development, but it seems to be in the right direction.

The XHTML documents 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. Our WEB books use exactly the same XHTML documents as in our EPUB books.