I don't know when I first heard of the Laurie R. King's Mary Russell series, but it was probably browsing the audiobook sestion at a library in the mid-1990s. I saw The Beekeeper's Apprentice, the first in the series, and I admit I was a unconvinced by the concept. A story featuring an older, retired Holmes appealed to me, but the idea of pairing him with a 15-year-old girl protege concerned me. This may have been a new concept for versions of Holmes, but as a comic book reader, "teen sidekick" is a tired trope that I have run across dozens of times.
But I took a chance, and am certainly glad that I did. That was a terrific book, and I re-listened to it with my wife on a summer vacation later that same year. The Holmes-Russell dynamic was powerful, and the books captured both the tone and the energy I was looking for. The next book, A Monstrous Regiment of Women, was not as much to my taste, but the third, A Letter of Mary, restored my confidence in the series. Exploring Russell's interest in religion and theology was an interesting place for the series to go, and this part of Russell's character remains with her as the series progresses.
I like all of the next six novels in the series to one degree or another, with The Moor and The Language of Bees among my top choices. The last two that I've read in the series have been good but not great. The God of the Hive was the first book I ever reviewed on this site back two years ago, and I read The Pirate King in the middle of last year. I am one book behind in the series, not having read last year's Garment of Shadows yet.
This series is by far the most in-depth I've gone in the Holmes "expanded universe." The mysteries in these novels are fine enough, but the strength is the world that King has moved Holmes and Russell into, and her characterizations of both. Her Holmes is in the spirit of the original, a reasonable extrapolation.
I will also point out that the character of Mary Russell has a very nice presence in social media for a fictional character, and I find her Twitter feed entertaining.