This book is written by two experienced SM people, one of whom is also a psychotherapist and a sexologist, and it does a very good job of covering the basics (and some of the not so basics) of SM. It is written in a readable, straightforward style.
It covers the basic SM safety practices of limits, consent, negotiation and safewords. It also covers how to do basic SM practices such as whipping, spanking, bondage and hot wax.
It includes good material on finding partners and on the SM scene, although since it was written the newsgroup alt.sex.bondage has changed to soc.subculture.bondage-bdsm
It has exceptionally good material on spiritual SM and on tantric SM.
There are shorter books on the subject and longer books but this is a good medium sized book.
Other good books in this area include "Screw the roses send me the thorns", "SM 101", "The Sexually Dominant Woman", "Learning the Ropes", "Safe Sane Consensual and Fun", "Ties That Bind", and "SM Classics".