Author (if given). Chapter Title (if using single book chapter). In: editor(s) (if using single book chapter). Book Title. Edition if applicable. Publisher Name; Copyright year:URL. Accessed Date.
Example:
Bigby M, Corona R, Szklo M. Evidence based dermatology. In: Wolff K, Goldsmith LA, Katz SI, Gilchrest BA, Paller BA, Leffell DJ. Fitzpatrick’s Dermatology in General Medicine. 7th ed. McGraw-Hill; 2011:http://www.accessmedicine.com. Accessed September 14, 2011.