MLA = Modern Language Association
The MLA understands that the world of citing electronic information is fluid (phone call, April 21, 2011).
Usually, you can use the style for citing a print book and then just add the additional information to indicate its electronic origin and when you accessed it, as shown below.
The point to remember is to include enough information for others to be able to find your source.
Book available only on the web or downloaded to an e-device
Book that has been scanned and put on the web (e.g., from Google Books or from The Complete Works of Charles Darwin Online)
See also the online example from the source below:
APA = American Psychological Association
Note that APA style requires you to add the type of e-device, where you retrieved the e-book, and the URL or doi.
These examples come from APA Style Blog (September 15, 2009), and Publication Manual of the American Psychological Association, 6th edition, 2010