Well, hell. He was one of the greats. His Parker books, written under the name of Richard Stark, are some of my absolute favorites. They're perfect gems, and textbooks on how to write tough, hard-boiled prose.
This is a stunning and heartbreaking loss to crime fiction.
The NY Times obit mentions that one reason Westlake used so many pseudonyms was "to combat skepticism over his rapid rate of writing books, which at some points reached four a year." Pretty sobering thought for those of us who are struggling to get one out a year, huh?
I best get workin'.