From Wikipedia - Doomsday A Study of Conversion, Proselytization, and Maintenance of Faith is a sociological book based on field study of a group of Unification Church members in California and Oregon. It was first published in 1966 and written by sociologist John Lofland. It is considered to be one of the most important and widely cited studies of the process of religious conversion, and one of the first modern sociological studies of a new religious movement. While a student at the University of California, Berkeley, Lofland lived with Unification Church missionary Young Oon Kim and a small group of American church members and studied their activities in trying to promote their beliefs and win new members for their church. Lofland noted that most of their efforts were ineffective and that most of the people who joined did so because of personal relationships with other members, often family relationships. Lofland published his findings in 1964 as a doctoral thesis entitled "The World A Field Study of Cult Processes", and in 1966 in book form by Prentice-Hall. The book introduced the expression "doomsday cult" to the English language and since then it has been commonly used in various contexts.
Very interesting read if you want to learn about the topics of the title (conversion, proselytization and maintenance of faith).
The book, at least the edition I have, follows the early years of the DP's (Moonies) in the USA. This section is very interesting as an example of "grounded" sociology and being attentive to detail of the inner workings of the group. The fact that the group got so popular ten years later, enables Lofland to write an appendix describing the explosion and expansion of the group in a more standard sociological perspective. This is also a very good read (maybe even more interesting).
If you have any interest at all in the sociology of religion, you have to read this for the same reasons as you do When Prophecy Fails (qv). The added spice here is the presence of the Moonies, establishing a bridgehead in the US through the sincere efforts of a wildly divergent group of American people.