This is *Maisie Dobbs by Jacqueline Winspear* - the first book in the beloved Maisie Dobbs series.
Published *2003*, it won the *Agatha Award for Best First Novel* and was a New York Times bestseller. The cover you have is the original 2003 edition - the young woman in the cloche hat is Maisie.
Here is a summary in English:
*The Heroine:*
Maisie is not a typical detective. In 1929, she is a psychologist and investigator in London, one of the very first women to run her own private inquiry office.
But her story starts much earlier.
Maisie was born poor, south of the Thames. Her mother died young. Her father, Frankie Dobbs, is a costermonger who sells vegetables from a cart.
At *13 years old*, Maisie goes "into service" as a maid in the grand Belgravia mansion of *Lord and Lady Rowan Compton*.
Lady Rowan is a suffragette. She quickly sees Maisie is exceptionally bright - Maisie sneaks into the library at night to read philosophy and science. Lady Rowan becomes her patron, along with her friend, the mysterious doctor and investigator *Maurice Blanche*.