Robert A. Maguire (1921-2005) was an American illustrator best known for his crime noir pulp book covers painted in the 1950s and '60s. His art defined the detective genre, and the over 600 covers Maguire created during his career made him one of the most important illustrators of the latter half of the twentieth century.
Like most illustrators, Maguire relied upon photographs of his models for his illustration work, and luckily, much of his reference has been preserved. To see how this artist used his reference, check out R.A. Maguire Cover Art, the only website authorized by the late Robert Maguire. At the site, you can see images of his printed covers, his original paintings, and the photographs from which Maguire worked.
There is also a great interview with Maguire in the American Art Archives which reveals some of the behind-the-scene action of the illustration world. It is a fun read, and I've included some of Maguire's remembrances of working with models below: