Marion Lane, book 1: Marion Lane and the Midnight Murder
T.A. Willberg2020

They were a band of mysterious private detectives who lived beneath the streets of London in a labyrinth of twisted tunnels and ancient hallways, the entrance to which no one had ever found. The Inquirers were something of a myth, a whispered legend that may or may not exist, depending on whom you asked. They were like ghosts, some said, these sleuths who guarded the city...

London, 1958:

Elaborately disguised and hidden deep beneath the city's streets lies the world of Miss Brickett's, a secret detective agency, training and housing the mysterious Inquirers. From traversing deceptive escape rooms full of baited traps and hidden dangers, to engineering almost magical mechanical gadgets, apprentice detectives at Miss Brickett's undergo rigorous training to equip them with the skills and knowledge they will need to solve the mysteries that confound London's police force.

But nothing can prepare 23-year-old apprentice Marion Lane for what happens after the arrest of her friend and mentor Frank on suspicion of murder: he has tasks Marion with clearing his name and saving his life. Her investigation will place Marion and her friends in great peril as they venture into the forbidden maze of uncharted tunnels that surround Miss Brickett's.

Being discovered out of bounds means immediate dismissal, but that is the least of Marion's problems when she discovered that the tunnels contain more than just secrets...