This is the only Book of Mormon speculative fiction written by a sitting general authority that I know of. Originally published in serialized form, it was later issued as a 111-page book; it’s easily readable in a couple of hours. Get it on PG.
It tells the story of Corianton (obviously), Korihor, Isabel, Alma, etc. and provides an interesting vision of what the details of their stories. I personally don’t spend enough time really imagining stories from The Book of Mormon (if my mental vision of the story of The Book of Mormon had the same production values as my mental vision of Narnia or Middle Earth, I’d have something), so it’s fun to get inside Roberts’ imagination. Corianton is presented as a sympathizer of Korihor, and the portrayal of Korihor’s debate with Alma is a high point of the book. Roberts manages to expand on the original narrative and make it feel realistic without inventing much.
Ardis Parshall wrote a fascinating paper for the Mormon History Association about the history of Corianton (available here on her blog Keepapitchinin). I recommend it for anyone interested in more detail about the plot, virtues, and failings of the book. She notes that it was later developed into a play, a short-lived Broadway show, and a 1931 movie, “Corianton: A Story of Unholy Love.” (In case there was any doubt, the absolutely hilarious Deseret News review of the movie places it somewhere in the vicinity of “so bad it’s good.”)