I went through an FLDS book binge a couple of years ago, love them all:
memoirs from women that left the FLDS cult, all give good info on the group:
Escape (best of the three)
Stolen Innocence
Shattered Dreams
19th Wife is a great novel/historical book about the FLDS
Girl Meets God by Lauren Winner is a fantastic memoir, she goes from agnostic to orthodox jew to episcopalian.
Travelling Mercies by Anne Lamott is a Christian memoir
The Red Tent is a fantastic novel (Jewish/OT Christianity)
and I am a total sucker for all of Beverly Lewis' amish books. I've read them all, most are part of a trilogy, but sometimes up to 5 books. Abram's Daughters series was my favorite. These are very quick reads but she gives good info about the Amish.
http://www.goodreads.com/book/show/118938.The_Covenant_The_Betrayal_The_Sacrifice_The_Prodigal_The_Revelation
I just finished the Rose trilogy and it was one of her better ones.
http://www.goodreads.com/book/show/7879046-the-thorn