Crossing the River Styx — Kristine Kathryn Rusch

Edith never wanted to visit the caves. But Frank insisted. Now, Frank lay in a pool of blood in those caves, and Edith wants to know why.
When Albert sees Edith emerge from the caves covered in blood, he has no idea what to think. He barely recognizes the wealthy newlywed who arrived just days before in the brand-new 1926 Chevrolet Phaeton he so admired.
Together, they must find the truth behind Frank's murder and the secrets hidden deep within the Oregon Caves -- or risk becoming victims themselves.

“As usual, a very good story from Ms. Rusch.”
-- Little Big Crimes