The best way to read an eBook file is directly on an eReader device (like an iPad, Kindle, or Nook). However, not everyone has one of these tablets or a mobile device. Are these people just out of luck?
One of the best things about eBook formats is that they are available for almost every device, including your desktop computer. There are several free applications that allow you to view eBook files right from your own PC or Mac.
One of the most popular programs to view ePub files on a computer is called Adobe Digital Editions.
Which you can download here: http://www.adobe.com/products/digital-editions/download.html
Amazon also provides a free solution for viewing .Mobi (.azw or .prc) files directly on your computer.