Besides the WMT series, probably one of my favorite examples of this is Poirot and Marple.
So it's a anime adaptation of different Poirot and Miss Marple stories produced by OLM (Pokémon), featuring the new character of Marple's great-niece Mabel as a bridge between the two characters.
It's surprisingly pretty faithful to the original stories. Mabel being there doesn't change the plot since she normally just says small lines from secondary characters that were already there in the novel, she really just serves as a audience surrogate.
Also the opening song is by the king of city pop Tatsuro Yamashita which is pretty freaking cool.