A very objective history of a religion that many Americans simply don't know too much about, stereotypes, politicians, and musicals notwithstanding. Filled in a lot of gaps in my knowledge.
Crams a good deal into some 200 pages - from the early historical struggles of the church and how it gradually became more integrated into American life. Discusses its controversies (polygamy, early territorial clashes) as well as its similarities to 'main-stream' Protestant Christianity - ethics, family life, missionary work, etc.
Also has a very nice bibliography, from which I will gleefully steal further suggestions.