Overall I was not impressed. I relatively recently (this past summer) read Brady Udall's book The Lonely Polygamist and was startled to come upon this idea that we should feel sorry for the man who takes multiple wives; that somehow these men are victims of this situation and end up with more responsibilities than they can handle.
That theme was also apparent throughout The 19th Wife; apparently we should all feel very sorry for the old men who essentially kidnap and rape young girls because ultimately those girls grow up to be demanding women.
Besides this very offensive thread, I found the main (modern) character of the book, Jordan to be annoying, the whole mystery of his dad's murder to be a waste of time and not very plausible, and generally poorly written.
The best part of the book were the snippets of Ann Eliza Young's book and the other "source" materials that were used for the fake MS thesis. I was extremely disappointed to discover that these were simply fictional materials. I am putting Young's book in my "to read" list, I am curious how much Ebershoff made up and how much is in fact based on Ann Eliza's story.