I'm a graduate from Wharton and a professional marketing consultant. When I first began doing marketing professionally, I longed for an integrated theory to guide me in whatever marketing challenge I would encounter. I started by turning to some very general marketing books ("The Portable MBA in Marketing" was one), but was quickly turned off by the stodgy tone of these books and the excess of information that beared almost no relevancy to everday marketing challenges! It seems the only thing classical marketing was albe to suggest was to find out as much as you can about the customer and then profile him, segment him, test out concepts on him, and keep doing this until you've found something that works.
That approach seemed too brute force to me, so I continued looking around for my grand unified theory. Not at all thinking I'd found my golden chalice, I noticed George Silverman's "Word-of-Mouth Advertising" in the bookstore and was intrigued by the prospect of sysmetically generating word of mouth.
After reading it, I realized I'd finally found my theory. In fact, what I considered my ultimate marketing theory was only the foundation for this book! Silverman then went on to use these ideas to discuss how to create systematic word-of-mouth campaigns! The grand theory, by the way, is simply modeling the customer buying process--not a huge innovation at first glance, but when presented by Silverman it becomes clear just how overlooked an idea it is. I've seen the customer buying process in marketing textbooks before, but it was never presented as the foundation for all marketing everything as, in my opinion now, it should have been.
The remainder of the book discusses how word of mouth works, the different levels of word of mouth, and plenty of other ideas. It tells you why people spread word-of-mouth (basically being so over-satisfied with a product/service that they feel compelled to talk about it) and how to help them do it (give them tools to quickly contact their friends).
In the end, this book makes me feel like a better professional because now I have an intellectual basis for approaching all my projects, rather than just "instints", "intuition" or other pools of ideas whose reliability and accuracy is somewhat flimsy.
I hate to say it since I'm basically talking to competitors here, but if you're in marketing, you pretty much need to buy this book.