His first big success was It Can’t Always Be Caviar which is a picaresque spy novel set during World War II, later followed by books like Dear Fatherland, set in Berlin shortly after the building of the wall, and And Jimmy Went to the Rainbow, about an assassination in World War II and the current trade in biological weapons. While critics saw an Austrian Heinrich Böll or Günter Grass in the making at the start of Simmel's career, his work drifted too much to popular fiction which made some critics dismiss him as a lightweight. Personally I wouldn't mind being such a lightweight - not if it includes an enormous audience in the German-speaking world, selling about 75 million copies of more than 30 novels worldwide! Many of his books have also been made into movies, some of which I've watched, and all of them with utterly depressing, sad endings.
While I never read any of his adult novels, I loved the books he wrote for children. Actually he wrote five, though I only know three of them. And, unfortunately, none seems to be available in English translation. It's not quite as bad as far as his adult novels are concerned. A selected few were translated, though if you try to get your hands on a copy, good luck! Simmel's books are still going strong in Austria and Germany though and a few have even been published for Kindle in recent years.