I just happened to pick up these books about Sherlock Holmes in a recent library run.
The first is a fun romp, with a MC who devises an observation routine (T.O.A.S.T.) and becomes a spy.
The second resets the Sherlock Holmes story to modern day, with a younger Holmes and Moriarty as roommates in an exclusive boarding school. Moriarty is a predictable teenage jerk.
The third is darker than my two kiddos generally read, and my daughter described it as "intense." It's not for a young middle grade audience; in fact, it's probably somewhere between MG and YA. Later books in the series are definitely YA.