At the time of writing this there are two main eBook formats which we are producing. In a year’s time it could well be different, and we will change with the technology. A third format, the PDF (Portable Document Format) created by Adobe, we regard as a legacy format although we still produce an eBook version along with some other older formats.

The two main formats, EPUB and MOBI, differ from Adobe’s PDF in that they allow the text to flow without page restraints. To explain this we can start with a PDF. If the PDF was created with 150 words on page one, then there will always be 150 words on page one, even if the eBook reader cannot fit all of that page onto the screen at the same time with the font size still being comfortably readable. The user may well have to scroll from right to left to read a line and scroll down to finish the page. This can become quite a nuisance when reading a long book.

I’m going to call EPUB and MOBI the real eBook formats. This is because they do not work with fixed page sizes. They may well, in fact usually do, put in page breaks at the end of chapters and such, but the number of words on a page is controlled by the user when he decides how big he wants the font to be. Starting with the default font size the eBook reading software will adjust itself to the screen size it is being displayed on. The screen will contain one page of text that fits that screen. If the font size is changed the software will adjust the word count to fit the page again using that font size.

Now we can look at the differences between the EPUB and MOBI formats:

EPUB
This is an open source format used by many of the leading eBook retailers like Barnes & Noble, Kodo and Apple. The file, which usually ends with .epub in its name, is actually a collection of HTML-like files zipped together in a single package. In fact you can look inside the package with some unzip programs.

MOBI
This is an Amazon proprietary format. In 2005 Amazon bought a French company called Mobipocket which owned an eBook format with the same name. This allowed Amazon to sell eBooks which were not readable on anything other than their Kindle© Reader hardware or software. Unlike EPUBs you can’t look inside the MOBI files using standard unzipping software.

Both of these formats have an option to combine the eBook file with DRM (Digital Rights Management) which is best described as a password protected file to stop the eBook being lent or given to another person. We will talk about DRM on another page.