I love girls who solve mysteries! I was a big fan of Nancy Drew when I was younger, I loved reading about Theodora's adventures in Theodora and the Serpents of Chaos, and now I love Gilda Joyce and Enola Holmes, both lead characters in two great series for middle readers.
The first book, Gilda Joyce: Psychic Investigator is written by Jennifer Allison follows 14-year old Gilda, a girl convinced she has psychic abilities and loves to people watch. She somewhat rudely invites herself to San Francisco to spend the summer at her mother's cousin Lester's home. Once there, she finds herself befriending Lester's daughter Juliet, also 14, and the two of them pair up to solve the mystery of Juliet's Aunt's death years ago. The girls spend their time in a house they believe is full of ghosts, try to figure out a way to get into the house's tower, and learn about each other as they go. Though they originally believed they were incredibly different, by the end they aren't so sure.
Gilda Joyce was a great character, though some of the language was a bit "advanced" for some middle readers. Nothing horrible or disturbing, just mature. I would put this one at 10/12 and up. She has two additional books she is starring in, those being Gilda Joyce and the Ladies of the Lake and Gilda Joyce: The Ghost Sonata.
The second strong female book series I have enjoyed lately has been that starring Enola Holmes, much younger sister of the famous Sherlock Holmes. In her first installment, The Case of the Missing Marquess, Enola is stunned when her mother disappears without warning on Enola's birthday, leaving behind only a few gifts and no note. Her older brothers arrive to assist Enola is finding her mother and when they realize that may be more difficult than anticipated, her eldest brother arranges for Enola to be sent away to a boarding school. Extremely upset with the news, Enola decides to take the mystery of her mother's disappearance into her own hands, before she gets shipped off to school.
Though not as quirky as Gilda, Enola definitely is a strong character, with very quick wit.... nice for the time period this book takes place in. This is another great selection for middle readers that love mystery!