tl;dr: The 'Enola Holmes' books are an engaging, entertaining and empowering take on the Holmes mythos which will be a hit with girls in their tweens.
This series is a set of six short books set from the perspective of Sherlock Holmes's younger sister, Enola Holmes. The series starts with their mother disappearing on Enola's fourteenth birthday. Mycroft and Sherlock are not interested in pursuing their mother's disappearance due to an old family quarrel and want to put Enola in boarding school. Enola has been bought up by her mother to be a free and independent thinker. She refuses to be part of the patriarchal and misogynistic system of the 1900s, runs away to London and tries to find her mother on her own. Her struggles to survive in London; her attempts to help others by using her powers of intelligence and deduction and her constant brushes with her brothers form the stories of the rest of the series.
Each of the books has a central mystery at its core that Enola has to solve. Typically, these are focused on women and their problems in the era. For example - women being forced to wear corsets, thrown into arranged marriages, being robbed etc. Enola solves these cases using deductive reasoning just like Sherlock Holmes does except that Enola uses her knowledge of dresses, flowers, habits of ladies as opposed to Sherlock's ashes, obscure trivia and human behaviour. Not that Enola's ability is restricted to only these topics, but these are the ones where Sherlock is at his weakest. I found this contrast quite creative. Enola also has to avoid her brothers while solving these cases since she is worried that they expect her to be a prim and proper lady.
Enola grows as a character (and to a lesser extent, Sherlock and Mycroft) from the first book to the last one. Initially, she is unsure of herself. She does not yet know her talents but manages to survive due to her wits. But as the books progress, so does her confidence in her abilities. She takes on more complex cases, is willing to put herself in danger, learns new skills and becomes the fiercely independent woman that her mother expects her to be. Reading about her and the way she deals with her problems is easily the best portion of the books. Sherlock and Mycroft too, grow a bit, as they slowly realise how women are being treated in their age as well as Enola's brilliance.
The books are of a decent length from a Young Adult's perspectives. I found the writing to be good as well as the pacing. The first couple of books tend to be a bit slow, but they become better from the third one onwards. There are a few subjects that are mentioned in passing like death and violence, prostitution and other vices which do not make this suitable for 9-10 years but will be fine for tweens or early teenagers, with proper guidance from their parents.
Overall, I liked the Enola Holmes books. They are an excellent addition to the Holmes universe.