These have been blogged about elsewhere, but the University of California press has published online two volumes of their Backstory series.
(The text below comes from an email from the good folks at the BlueCat Screenwriting Competition, which alerted me to the books.)
The collection includes screenwriters Leigh Brackett, Richard Brooks, Betty Comden, Adolph Green, Garson Kanin, Dorothy Kingsley, Arthur Laurents, Ben Maddow, Daniel Mainwaring, Walter Reisch, Curt Siodmak, Stewart Stern, Daniel Taradash, and Philip Yordan.
These screenwriters are behind such films as: The Big Sleep, Rio Bravo, Singin' in the Rain, Adam's Rib, Rope, West Side Story, The Asphalt Jungle, Invasion of the Body Snatchers, The Wolf Man, Rebel Without a Cause, From Here to Eternity, Johnny Guitar, and Cat on a Hot Tin Roof.
This e-book contains interviews with Jay Presson Allen, George Axelrod, Walter Bernstein, Horton Foote, Walon Green, Charles B. Griffith, John Michael Hayes, Ring Lardner Jr., Richard Matheson, Wendell Mayes, Irving Ravetch, Harriet Frank Jr., Arnold Schulman, Stirling Silliphant, and Terry Southern.
These scribes wrote screenplays for such films as: Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb, In the Heat of the Night, Norma Rae, Anatomy of a Murder, Twilight Zone: The Movie, MASH, Rear Window, To Catch a Thief, The Man Who Knew Too Much, and The Wild Bunch.