Once you use an eBook reader or eBook apps in a Tablet you will realize that reading a book in a desktop is like driving an Ambassador car. When an Audi or BMW is offered to you within your budget, who wants to drive a M800.
But there can be some occasions where you are forced to read a book using a desktop or a laptop. There are many apps available to help. Adobe Digital edition can be used for ePub format or better use Calibre which supports all the formats.
Calibre is a free cross platform eBook Management tool, convertor and an eBook reader bundled in to one. It supports all popular eBook formats and can be used as an eBook reader for all of them. It is available in Windows, MAC and Linux, and comes with a good interface with lots of features and it is free too. So there is no need to look elsewhere if you need an eBook reader in a desktop or a convertor to convert from one format to another.