The books on this site are currently all ebooks. In order to read them, you may need to download a file or app from a website or Play Store.
If you have a Kindle, or you’re an Amazon customer, you need a file with the suffix .azw3; .azw; .kf8. If you don’t have a Kindle but want to buy from Amazon, you can download the software from Amazon onto your PC, laptop, tablet or phone. The location of the download varies from country to country – you may need to poke around on the Amazon website a little.
If you have an Android smartphone or tablet, or a Blackberry, download eReader freeware. There are many different types available in the Play Store on your device. eReaders use the .pdb suffix,
If you have a Kobo eReader, download a file with the .epub suffix.
If all else fails, retailers often offer a .pdf file (Adobe Acrobat) file, which will run on just about anything.
Source: Wikipedia for a comparison of file formats.