The Secret of the Three Fates by Jess Armstrong
The Secret of the Three Fates picks up a few months after the first in the series, The Curse of Penryth Hall, in October of 1922. This one probably works as a stand alone, but you'll have a much better feel for the characters and appreciate what we learn about their backgrounds here.
This time around, Mr. Owen and Ruby are at a remote manor house in Scotland, supposedly to purchase illuminated manuscripts. Mr. Owen, however, has lied to Ruby and the two of them are actually there to attend a séance- Mr. Owen is hoping to speak to his son, Ben, who died in the War. The séance, hosted by three mediums going by the name of The Three Fates, is frighteningly real. One of the mediums, seemingly possessed by a spirit, delivers an ominous message:
There’s nowhere on earth you can hide from the dead. We have not forgotten … we shall not forgive. The dead know what...