I have several of his books in my "read these" pile. He died New Year's Eve on the way out to dinner with his wife. Here is the NY Times obit.
A piece of trivia: Remember Richard Bachman, Stephen King's pseudonym? King got that name when he was asked to come up with a made-up name on the spot. He had a Bachman Turner Overdrive record on his turntable, and his eyes landed on a Richard Stark novel on his desk, and came up with Richard Bachman! Richard Stark was one of the many pseudonyms Donald Westlake used.
Sad day for his fans.