An M.D. is an important degree, but the American Psychological Association style manual doesn't include degrees in the references list or in-text. When citing a book author who happens to be an M.D., use the same style as you would for any other author.
References Section
In the references list, enter the M.D.'s last name followed by a comma and then her first and middle initials followed by a period. Enter an open parenthesis, type the year followed by a closed parenthesis and then a period. Type the name of the book in italicized sentence case followed by a period; in sentence case, capitalize only the first letter of the book's name. Enter the city and state where the publisher is located followed by a colon and then enter the publisher's name.'},{'content':An entry by an M.D. in your references list should look like this:
Brown, P.A. (2013). A physician's diary. New York, NY: Medical Publishers.
If the book is written by two or more doctors, format the entry as follows:
Brown, P.A., Michel, J.M., & Knight, R. (2013).
In-text Citation
To cite an M.D. book author in-text immediately following the information cited, in parentheses enter the last name only followed by a comma and the year that the book was published.
For example: (Brown, 2013), or (Brown, Michel, & Knight, 2013).