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.
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.
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).
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).