For an electronic version of a book (an e-book) include the author, date of publication, title of work, and then either a doi number or state "Retrieved from" and list the URL. A 'doi' number is a Digital Object Identifier that uniquely identifies the content. The following examples are from the APA Style guide, 6th edition, on page 203.
Shotten, M. A. (1989). Computer addiction? A study of computer dependency [DX Reader version]. Retrieved fromhttp://www.ebookstore.tandf.co.uk/html/index.asp
Schiraldi, G. R. (2001). The post-traumatic stress disorder sourcebook: A guide to healing, recovery, and growth [Adobe Digital Editions version]. doi:10.1036/0071393722
Use the same citation format as for a print book and then include, as applicable, the name of the website, the medium and date of access. If there are no page numbers use “N. pag.” after the publication information. The example below is from this informative e-book guide at St. Edward's University:
Welch, Kathleen E. Electric Rhetoric: Classical Rhetoric, Oralism, and a New Literacy. Cambridge: MIT, 1999. netLibrary. Web. 21 Oct. 2004.