The NYU Libraries guide to Citing Sources in APA STYLE has a section on citing “electronic” sources such as web sites, e-text sections and chapters, blogposts, podcasts etc. For example, here is a citation for a popular “point of care tool,” UptoDate:
Borody, T.J., Leis, S., Pang, G., & Wettstein, A.R. (2013). Fecal microbiota transplantation in the treatment of recurrent Clostridium difficile infection. In T.W. Post, P. Rutgeerts, & S. Grover (Eds.), UptoDate. Available from https://arch.library.nyu.edu/databases/proxy/NYU04665
Note, the use of “Available from” indicates that the URL will lead users to a download site rather than directly to the chapter. This is a distinction to be made, compared with “retrieved from” in this example:
Centers for Disease Control and Prevention. (2014, April 2). Centers for Disease Control and Prevention. Retrieved from http://www.cdc.gov/
The difference is that the first example is an NYU-licensed resource, so an outside user would be led to a site but not necessarily be authenticated to retrieve the full text. “Retrieved from” indicates that a direct download is likely, in this example a freely available web site, like the CDC (Centers for Disease Control and Prevention).