Westlake wrote in a few genres, but primarily comic crime novels. Intricately plotted and very, very funny, his books are populated by quirky crooks, inadvertent spies, and assorted ne'er-do-wells. Mostly without a conscience, the Westlake hero sometimes manages to not only make us laugh but also make us like him.
He also wrote a few more serious novels. Kahawa, which takes place in Idi Amin's Uganda, has a comic crime premise but is a horrifying tale or insane despotism. I asked Westlake at a panel discussion about the genesis of that book, saying "It seems like it could almost be 'Dortmunder Goes to Africa' in the beginning, but it's an entirely different book." He said he started off writing another comic novel, based on a true incident, but that the closer his research took him to what was going on in Uganda the more he realized that wasn't the book this was going to be. I highly recommend it.
Another serious book of his is The Ax, about a middle aged man who turns to murder after being laid off from his long time job. The reader gets into the protagonist's head to an alarming degree, to the point where you want him to get away with the murders if only to get another job.
Some of his books are about writing process and may have additional interest to writers because of that. Adios, Scheherezade is an ingenious albeit ultimately flawed novel about writing pornography. A Likely Story is about an author whose book encounters a lot of problems between initial submission to his publisher and publication and it's unclear whether it ever will hit the bookstores. The author's frustration with the progress (or lack of it) with the book is interwoven with a story of his divorce and new romantic interest to make a fun, interesting, and quirkily satisfying novel. The Hook - the best of Westlake's books about writing imo - tells about a murderous plot between two novelists and schemes that spin out-of-control in a Strangers-on-a-Train-esque fashion.
Westlake was an amazingly prolific author, having written over 90 books. I read a whole bunch of them in my teens and twenties. After that, I generally read Westlake's books from the "new books" section of my local library as they came out. But I also check the W section in the Fiction shelves, because every once in a while I find one of his that I somehow missed when it was published and have a little thrill to think there's a new (to me) Westlake to read. That's what happened to me last week with Money for Nothing, the story of a guy who is a "sleeper" spy and doesn't know it.
My days of finding new Westlakes are numbered. The last of his books will be out in April of this year. Rest in Peace, Donald Westlake. You gave a lot of people a lot of entertainment, myself included.