Thief in the Night — Abie Longstaff

The third book in The Trapdoor Mysteries, a series about Tally, a code-breaking, animal-loving servant girl and her best friend, a squirrel named Squill, who solve mysteries with the help of a secret library...

Orphan Tally is the servant girl at Mollett Manor - and she's also the Secret Keeper of the magical library hidden beneath a trapdoor, underneath the manor's grounds. Along with Squill the squirrel, she uses the enchanted books to solve crimes.

So when ghostly noises start haunting the lady of the manor in the middle of the night, she's terrified and turns to Tally for help. But can the library help Tally catch something that no one can see?