There are a couple of different ways to read eBooks. To read the books from this website on your computer, the most versatile program I have found is Calibre. This is a free program but is very powerful. If you do download this file and use it, please send them a donation. Remember, a workman is worthy of his wage.
If you want readers you can put onto your cell phone or other reading devices, here are some choices:
For details on other file extensions visit this Wikipedia Page. If you scroll down a bit you will find a very helpful chart comparing all the different types of ereaders.
It is also possible to sideload .epub and .mobi files onto your Kindle and Nook by “sideloading” them from your computer. Simply plug in your ereader into your computer. Then, navigate to the appropriate folder and drop the files into that folder.