OK I didn't know amateur used to have a different connotation--it's important he's an amateur in that he's a professional but unlike the poorer sportsman he doesn't need to make his money through sports it's not his profession! His profession is crime
I did wonder why everyone seemed to be into what this amateur did.
OHHH THIS IS GREAT!! I never thought of that!! The Amateur part of amateur cracksman is that he's an amateur cricketer because he doesn't use his cricket to get any money!!! Fascinating thank you!! That's very clever Mr. Hornung very clever indeed.