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
