So this category sort of replicates the favourite series and favourite book etc.. that we've done earlier, so I'm going to go with my favourite mystery writer. :)
That would be Barbara Hambly. Along with some well-known fantasy novels, Barbara also writes the Benjamin January series of books. The books are set in and around New Orleans in the 1830s. Benjamin is a free man of colour and the series opens when he returns from France after a very long absence. (Benjamin's Wikipedia entry: Mixed-race former slave, freed as a child by his mother's lover. He was trained in Paris as a surgeon, but works primarily as a piano player. He is very tall, and very dark-skinned, which is a significant impediment to his medical career in pre-Civil War New Orleans. He lived in France for many years, but returned to New Orleans when his first wife died.)
Barbara does her homework and the books are rich in history and atmosphere but that never gets in the way of the story-telling. Her characters are full and real and some of them you love, some of them you hate. The mysteries are well-plotted and definitely not just excuses for the story of Benjamin's life. The two twine wonderfully well.
Whenever I learn that someone enjoys mysteries, I immediately recommend this series. It's terrific!