I don't think the "friendly" book on the science of dreaming has been written, though both Van De Castle in his Our Dreaming Mind and Anthony Shafton in his Dream Reader have both attempted a recent summaries of dream science.

If you are willing to take on a little more time and study, I have three suggestions.

1. Harry Hunt's (1989). The Multiplicity of Dreams. Yale University Press.
This is on my TOP FIVE DREAM BOOKS list. Hunt's book is a little difficult at times as it is often a *reaction* to a lot of the dream science and theorizing that has taken place and he is inconsistent about who his audience is. Sometimes he carefully explains his thoughts and the experiments, while at other times he makes assumptions on the part of the reader that include having read a great deal of the literature on dreams and cognitive science theory. Still, it is at this time the best (intermediate) overall survey of dream science theory.

2. Moffitt, Kramer and Hoffmann (1993). The Functions of Dreaming. NY:SUNY press.
[For a little more detailed analysis of the literature, this is a really good book. Some areas are ignored or left out, but the ones covered are done so quite well.]

3. Ellman, Steven J. and Antrobus, John S. (Eds). (1991).The Mind in Sleep: Psychology and Psychophysiology. 2nd edition. New York: John Wiley and Sons, Inc.
[This is a little more advanced of a text, but covers a large range of topic in dreaming science and does so quite well. If there is a fault, it the dryness and objectivity offered. I often get the feeling when reading this that we don't really know anything about the science of dreams and all experiments up to the present have been very inadequate.]