Just to share from a tweet I received.
http://www.nasa.gov/connect/ebooks/index.html
NASA offer three different file formats to accommodate as many electronic reading devices as possible.
EPUB is the most common format for e-Books. If you use a Sony Reader, a Nook, or an iPad you can download the EPUB file format.
MOBI can be read by the Kindle and Kindle DX. If you use this device, you can download the MOBI file format.
PDF files can be read by almost all desktop computers. You will need to install Acrobat Reader from Adobe's website to view the PDF.
NOTE: CTRL-click to save file (Mac and PC).