May I suggest a trip around the world of detectives. You’ve already had a number from the U.S. and England suggested. How about Hawaii, pre-statehood of course, Earl Der Biggers and his Charlie Chan series, then move on to Australia with Detective Inspector Napoleon Bonaparte (Bony) by Arthur Upfield, and then up to China with Judge Dee by Robert van Gulik, and continue around the world.
Seconded, thirded or whatever for Dorothy L Sayers (Lord Peter Wimsey) and Lindsey Davis (M D Falco) and keep going with Ngaio Marsh - plenty more if you have only read ten of them. How about Emma Lathen’s John Putnam Thatcher stories? Good fun and you learn about some aspect of business or banking (at least in the early ones) 
Quick side note: For anyone who wants to know how many mysteries a writer wrote in x series (i.e. Margery Allingham), or what other authors wrote during a certain time period, etc. try Stop You’re Killing Me:
Apologies to anyone who already knows about this site. It is the anal retentive mystery lover’s friend. 
Yes, yes it was.
Thanks, everyone, for the great suggestions.
Nice site. Not perfect, of course. I looked at some of the authors mentioned here and found a big hole. None of the Ellery Queen short story collections are mentioned, even though story collections are included everywhere else I looked. And some new releases haven’t been added yet, like the new Edward D. Hoch collection I just bought.
Still a great site and an obvious labor of love.