Murder at the Mystery Castle — J.C. Eaton

Twenty-something neophyte detective, Marcie Rayner, has put one murder investigation to rest at Blake Investigations in New Ulm, Minnesota, when another case, emanating from the same source (her mother), lands in her lap. When Helena Heatherbrae, the wealthy proprietress of an iconic architectural structure east of Minneapolis is found dead, her cook is positive it was murder. To complicate matters, Marcie has just taken on another client who's convinced her fiancé is cheating on her.
As Marcie juggles both cases, she learns that the secrets hidden in the labyrinthine Mystery Castle are nothing in comparison to the ones stemming from her clients. Using her newly acquired skills as a detective, coupled with her own street smarts, Marcie navigates through a maze of off-beat suspects as she probes deeper into both cases. The caveat is when she finds her client's philandering boyfriend is linked to the Mystery Castle in a way she never imagined.
The closer Marcie gets to finding the truth behind Helena's death, the more she realizes some secrets are better left undisturbed.