(The five-stars here, of course, is relative to the genre..., not to Proust)
MacDonald's mastery of his craft is evident on nearly every page here - both in the writing, and in the construction and characterization and believablity. Everything I find lacking in science fiction, I find present in crime fiction... including a certain moral tenderness.
In the Ivory Grin, RM also deals with the issue of race - circa 1952 -- and though he writes as a white man, and pretends nothing else, he has an eye for detail and nuance that rings largely true. (One exception was the idioms used in the letter written to Lucy, early in the book). Macdonald was an excellent observer of human character and physionomy, linguistically inventive, and a patient constructor of plot.