Montalbano novels because I liked that character on TV and the paperbacks were in the bookshop but I never got into reading all of Poirot or all of Marple.
I have commented that each installment of the Yamamura series hints at the supernatural and that the novels also display the perspective of an sf writer but neither of these directions would have been the way to continue the series much though I like multi-genre experimentation. A novel set in the here and now can be about any aspect of the character's life or about other characters that he meets. Ian Fleming managed to do this with three James Bond short stories and with two thirds of one of the novels. I did not expect to reference Fleming when I began this post but he fits.