Personally, I adore the Mystery Shop genre. I might've become obsessed at the moment CLAMP started doing it, since I was already in the CLAMP fandom when the serialization began in Japan (and man, were raw scans hard to find in the old days! there were whole arcs of XXXHOLiC I had to "read" in Chinese scanlations and didn't know what they were about until they were published in Spain months/years later). So I see your point of them making it more mainstream in the West, although funny enough it wasn't MY first bite on the genre. I started liking it when I watched the PSoH OVAs when I was what, 19-20? And I always felt PSoH had to be somehow inspired on the Gremlins movies back in the 80's.

Can't rec any because you've already listed the manga/manhwa I already know :)