He got a lot done in his life. He had experience as a radio DJ and a freelance writer. He created with Terry Gilliam the satirical 1966 book The Cocktail People around the same time as the two of them were surviving jobs in an advertising agency (they got out with their souls intact). And in 1997 -- I just found this out -- Siegel wrote a book for his son Dylan before he was born, because he didn't know (due to his cancer diagnosis) if he'd live long enough to see Dylan be born. That was 10 years ago.
Godspeed, Mr. Siegel. It's always too soon with cancer, isn't it?