Julian Rathbone was a prolific author writing in a number of genres in addition to the crime novels discussed here.
Pluto published two of the the three Jan Argand novels. Strangely Watching the Detectives, the last novel was published first with Introducing Commisioner Jan Argand on the cover. This was followed by The Euro-killers the first novel. The correct sequence is
Given that Rathbone wrote another book called "Accidents Will Happen" and mentions "Olivers Army" in another book "Trajectories" I assume the naming is deliberate.