As the Seventies drew to a close, Stanley began to concentrate on more serious books as there was a distinct shift in the popular market towards thrillers and crime novels, particularly in the USA. Whilst Stan's serious books from this period stand up there with the best of them - problems with Publishing Companies conspired against him. His last book "Trance" was a return to the genre and considered by many (this reader included) to be his finest non-comedy work. Originally published under the nom de plume "Richard Kessler" in an attempt to disassociate the author from his Tobin legacy, it was finally published under his own name in 2006. Stan retired from writing shortly afterwards.
You could probably class all of these serious novels as being "The Revenge Series" - as each one deals with revenge as a central plot theme (even though there is no other connection between the books).
Quite why they ended up thematically similar I'm not sure, but it was certainly a plot device that Stan liked, and did a grand job of weaving a story around in all of these novels.