The ebooks are in Adobe Portable Document Format (PDF) compressed into one (1) .zip file for downloading purposes. To uncompress the files, you will need a zip extractor program.
If you are using Windows, Windows may be able to automatically extract files. Just double click on the downloaded .zip file.
If you are using another operating system, a free copy of a zip extractor can be found at http://www.7-zip.org.
You will need the free Adobe Acrobat Reader to read the eBooks. A copy of the Reader can be found at http://www.adobe.com/products/acrobat/readstep2.html.