This is an excellent introduction to a fascinating and budding field. Sociolinguistics is basically the study of how language interacts with society, and how it varies within and between societies. It is perhaps the more modern version of dialectology, though dialectology is still around, and Trudgill has written several other books in that field. But he fully understands the differences, and his dual expertise makes some sections of the book more interesting.
This book is one of the most readable academic books that you could pick up, especially in linguistics. Trudgill introduces sociolinguistics in ten chapters which each focus on the way language is related to certain other generally social factors. These include social class, ethnic group, sex, context, nation and five others. Trudgill is British, and therefore many of his examples have to do with Britain: the variation of English and the Celtic languages. He does not neglect looking at other Englishes, however, and this will be a definite advantage for the reader in Britain. However, other readers will not suffer, as his examples are clearly explained. Besides English and the Celtic languages of the British Isles, he also discusses countless other languages, raising interesting points and illustrating them with nice examples.
If you read this book, you will soon become an expert in basic sociolinguistics. You will find yourself using many of these examples in conversation, almost feeling that they were your own. It's a great way to get started learning about the field, and it is accessible to those already in linguistics as well as the casual reader.