Referencing electronic version of a print book in a reference list...
eg. DeLara, M. & Doyen, L. (2008). Sustainable management of natural resources: Mathematical models and methods. [SpringerLink version]. Retrieved from http://www.springerlink.com/openurl.asp?genre=book&isbn=978-3-540-79073-0
The in-text citation for this would be (DeLara & Doyen, 2008)
Here's another one:
Ardia, D. (2008). Financial risk management with Bayesian estimation of GARCH models: Theory and applications. [SpringerLink version]. doi: 10.1007/978-3-540-78657-3
In-text citation would be (Ardia, 2008).
If it's an electronic-only book, example as follows:
Stevens, K. (n.d.) The dreamer and the beast. Retrieved from http://www.onlineoriginals.com/showitem.asp?itemID=332
In text reference would be (Stevens, n.d.)
And, just for good measure, for a translation of a book:
Yinen, J. (2008). Stretching therapy: For sport and manual therapies. (J.Nurmenniemi, Trans.) Edinburgh, Scotland: Churchill Livingstone.
In text reference is (Yline, 2008).
It doesn't seem to say what you do about page numbers if there is a direct quote from one of these sources, though. I guess you would just put the page number from the electronic device on it. My e-reader (a Sony) does have page numbering, although I don't think that always correlates with the page numbers in the print version.
Anyway, this is all from the University of Queensland Library information sheet on References/Bibliography APA style. Retrieved 14/2/2012 from http://www.library.uq.edu.au/training/citation/apa_6.pdf
This page seems to be accessible even if you are not a UQ student, so you might find any other information you need there.
Or you could go straight to the source at http://www.apastyle.org/index.aspx
Hope this might be helpful to start with...