A mysterious man, trapped in a mirror of his private detective office, recruit a boy, 13, and his sister, 10, to travel back in time to the year 1939 and clear him of a crime he did not commit. To do this, they masquerade as adult detectives and roam Los Angeles, questing gangsters, and their molls as they seek to find the true murderer.
Jake and his younger sister, Darlene, while temporarily staying in a downtown residence in Los Angeles, come across a skeleton key to a door that strangely no one else in the building can see. Printed on the frosted glass of the door is the name, Archer Wolfe, Private Detective. As they enter, they find two rooms furnished with strange furniture dating back to the 1930s. Exploring further, they discover the ghost of Archer Wolfe trapped in a mirror on the wall. Convicted and executed for a murder he did not commit, the detective has one request: Venture through 1930s Los Angeles as private detectives and find the real murderer exonerate Archer Wolfe of the crime. However, they must accomplish this within a short amount of time because the ghost of the detective and his world of the 1930s will soon vanish.
Mystery of the Man in the Mirror fits as a children's book, a young adult fiction book, and falls into the mystery adult noir genre as well.

Thirteen-year-old Jake and ten-year-old Darlene have just moved into the Santa Fe Building Lofts on Los Angeles’ Sixth Street. They will be living in their uncle’s loft with their parents until they can find more permanent housing near their father’s new job. Facing a long, boring summer ahead while their parents are away at work, the two children begin exploring the old building in search of something to do. Jake comes across an old skeleton key and finds it fits the lock of a door to a private detective agency on the third floor. Jake and Darlene are shocked to find a man trapped within a mirror inside the office. The children learn the man is Detective Archer Wolfe, who was framed eighty years earlier for a murder he did not commit and received the death penalty. The last moment he remembers was his execution. Eager to be a part of the mystery, Jake and Darlene waste no time hunting for clues, determined to prove Archer’s innocence and clear his name. Jake and Darlene realize the skeleton key is the one link between the past and the present timelines and that they can use it to cross back in time at various locations around Los Angeles. But time is running out as they learn they only have a limited time to exonerate Archer and set things straight.