Like the other books in the "Oxford Introduction to Language Study" series this book aims to provide a "brief but comprehensive introduction" (blurb) to its topic. Generally, Spolsky suceeds admirably: he provides a dense but still easily readable account which can be used as a first introduction to the field but also to recapitulate the most important areas of sociolinguistics. As usual, the book contains a survey of key concepts (only 80 pages!), short readings from other relevant works and a selection of annotated references.
However, readers should be warned that the book is (because it is so short) necessarily rather dense. I regard this as positive, though. I'd rather read 70 pages than 300 with the same amount of information!