The Mystery of the Spirits - C. S. Poe

Antique dealer Sebastian Snow and Homicide detective Calvin Winter have been happily married for a year and a half. In that time, there's been nary a mystery in sight, and for a recovering sleuth like Sebastian, an uneventful life is exactly what he needs.

That is, until Calvin's lieutenant enters the Emporium and demands insight on a bizarre object known as a spiritoscope, hailing from the early days of the Spiritualism movement. Sebastian's extensive knowledge of Victorian curiosities leads him to consulting for the NYPD—putting him at odds with his husband. And as the bodies begin to stack up, so do the seemingly dead-end clues, which if Sebastian can't make sense of, might result in a whole lot more death.

Mystery, murder, and marriage... Sebastian's back.