Same here, not a fan of the rules, BUT I do like that Agatha pretty much broke them all - sometimes several in the same book (*cough* The Big Four).
Go Agatha!
I read online- "the 'No Chinaman rule' was a reaction to, and criticism of, racial cliches prevalent in 1920's English writing." And Knox made this statement in explanation of it-
"No Chinaman must figure in the story. Why this should be so I do not know, unless we can find a reason for it in our western habit of assuming that the Celestial is over-equipped in the matter of brains, and under-equipped in the matter of morals. I only offer it as a fact of observation that, if you are turning over the pages of a book and come across some mention of 'the slit-like eyes of Chin Loo', you had best put it down at once; it is bad. The only exception which occurs to my mind--there are probably others--is Lord Ernest Hamilton's 'Four Tragedies of Memworth.' "
Go Agatha!
"No Chinaman must figure in the story. Why this should be so I do not know, unless we can find a reason for it in our western habit of assuming that the Celestial is over-equipped in the matter of brains, and under-equipped in the matter of morals. I only offer it as a fact of observation that, if you are turning over the pages of a book and come across some mention of 'the slit-like eyes of Chin Loo', you had best put it down at once; it is bad. The only exception which occurs to my mind--there are probably others--is Lord Ernest Hamilton's 'Four Tragedies of Memworth.' "