Let's say you want...
We recommend the .epub version. But first, you'll need to install a compatible software like Nook's desktop apps or even the Firefox extension EPUBreader.
More .epub-compatible software options, some of which also read .mobis, can be found in Wikipedia's .epub software listing. Look around and experiment with different options. Find which one works best for you.
The .epub version.
We've found that a few Windows machines take our .epubs and, inexplicably, rename the .epub file extension .zip. None of our ebook files should come to you in .zip format. If you're seeing a .zip file, it's an error.
The easiest way to fix this is by renaming your file from XXXX.zip back to XXXX.epub. Then make sure you open the .epub file with an epub-compatible software or device.
There's an few options.
A .mobi, like an .epub, is a file extension designed especially for ebooks; but mostly for the Amazon Kindle.
You can read .mobi files on your PC or Mac with certain software; but we usually recommend using the .epub version in most instances unless you're transferring your book to a Kindle.
An .epub file is a type of file designed especially for ebooks and ereaders. It's most notably the format of choice for iBooks, iPads, iPhones, and Apple devices. Epub files do not work on Kindles. For that you'll need the .mobi.