Ebook requires Epub reading software, which you can download free at the following sites:

 FOR PC -

Mobipocket Desktop for PC and many cellphones:
http://www.mobipocket.com/en/DownloadSoft/ProductDetailsReader.asp

 EPUBReader add-on for the Firefox browser:
https://addons.mozilla.org/en-US/firefox/addon/epubreader/

Bookworm, an online program for storing and reading e-books:
http://bookworm.oreilly.com/

FOR MAC -

You can download Calibre which is for macs here:
http://calibre-ebook.com/download_osx

Or try Stanza
http://stanza.en.softonic.com/mac