You can read here to get the overview of the series. This book has a Terran happen to crash a plane, his companions killed, up in the mountains. A ghost named Callista helps keep him safe. She insists she's not a ghost, but is held in a dark place and can't reach her family and friends with telepathy but can reach him. She manages to get him to her father's castle where a kinsman of Callista's helps the Terran get more into the "overworld" to see things.
Callista's father comes home to her sister's call of help, but he was attacked on the way by catpeople and is paralyzed from the waist down. The kinsman insists on going to pick up Callista, but he agrees he's not very good at swordwork. The father is able to take over the kinsman's arm and use the sword. They head into the cave to save Callista.
A rather minor story, lots of odd bits stuck in, and no explanation of how the Terran has the telepathy. I loved these books when I was a teenager and continued buying them until MZB stopped writing them and others did, but they're going to the bookgroup this month, where nobody will want them and I'll put them in the Friends of the Library cart.