Intruders at Rivermead Manor — Kathryn Reiss

To help her family make ends meet, Kit takes a job helping an elderly lady at a rundown mansion called Rivermead Manor. When she discovers a hidden room in the old house, she is sure its secrets will make a great newspaper article. But then Jessamine, Kit's long-lost friend, appears and seems to be in trouble, yet she won't confide in Kit. Determined to help, Kit follows clues, but will both of these mysteries lead to a story that's bigger than she could have imagined, or will they stay hidden at Rivermead Manor?