Communing with Mr. Quin: The Shadow on the Glass by Agatha Christie

Harley Quin is ba-ack! But barely; he makes all but a fleeting
appearance in this light yet surprisingly robust tale of double
homicide rooted in a ghost story dating back to the English Civil
War. (If you wanted a refresher on the "Roundheads" versus the
"Cavaliers," you are in luck!) So grab some popcorn and watch along
with us and Mr. Satterthwaite as the romantic sparks--and
bullets--begin to fly....