"The Lion of Farside" by John Dalmas
This is a fairly straight-forward adventure/fantasy story. Our hero (Curtis Macurdy) is a country lad from Washington country, brought up on a farm, and intending to lead life as a farmer. When his uncle dies his aunt (not a blood relative) sets her sights on him - we already know she's a little odd, for instance she doesn't seem to have aged in the last 20 years, doesn't look a day over 20. They marry and move to another state to farm and live their lives together. She tells him about her world - which is a completely different world reachable only via 'gates' in particular places, and she begins to teach him how to unlock his magical powers. However, she is then kidnapped and forcibly taken back to her own world. Curtis then follows her, and the bulk of the story is Curtis's hunt for her and her attempts to get back to Curtis. It is a fun story, and doesn't end at all how you'd expect.