I'll admit it: The 19th Wife caught my eye because it's about polygamy, a subject bound to arouse curiosity (as the characters themselves observe!). On that front, the book delivers. It comprises two narratives: one, the story of Ann Eliza Young, one of the many wives of Brigham Young and eventually an anti-polygamy crusader, and the other, a murder mystery involving a modern-day polygamist sect, as told by Jordan Scott, a young man who was expelled from the group. The two narratives coexist uneasily: while the author tries to link them (for example, through a student who writes about Ann Eliza and appears in the contemporary story), they don't quite fit together.
The historical part is told via a wide variety of documents: primarily a fictionalized version of the real Ann Eliza's memoir, supplemented by memoirs from other characters, scholarly works, letters, and so on. Together, the documents weave a compelling tale, with the drawback that we are distanced from the characters. Ann Eliza's memoir was written with a political goal (to end polygamy) and her credibility is called into question; other characters, too, have their own agendas. Those who like unreliable narrators will be thrilled, while those who want to get to know the characters as people are likely to be disappointed. Either way, this part of the book does a great job of covering the early history of the Mormon church and showing how polygamy affected people's lives.
Meanwhile, the contemporary story is much faster-paced, full of snappy dialogue and modern slang. Here the problems are reversed: Jordan is a great character and very sympathetic (and it's nice to see a gay protagonist for once), but the plot doesn't go much of anywhere. He spends most of the book driving around to talk to other characters, supposedly looking for evidence to exonerate his mother, who's been accused of killing his father; however, he never has any leads and after awhile it started to feel to me as if he was actually hunting for a plot. I didn't like the supporting characters in this section: there's an annoying kid who gets far too much page time and a love interest who comes out of nowhere (nobody gets that serious that fast. Nobody). The end is poor on all levels. But again, there's a lot of information about a sect most readers probably know little about, and I found that to be worth my time.
Ultimately, I found this to be a fun book that will pull you in and educate you, and although it disappointed in some ways, I still recommend it.