How good are "For Dummies" books in Academic Study?
Those books are not only very useful, I would argue they are the best option in many cases, as they represent the best use of your time. You will find that, throughout your graduate studies, you will have to learn aspects of many different fields. In most cases, while it would be possible for you to embark on a thorough study I some other discipline, that would take weeks, if not months. These books allow you to quickly learn the basics, giving you a solid foundation of knowledge that you can expand with further study if necessary. 

Edit: this applies to topics only ancillary to your main field of research; you should not use this book on your research topic itself, for the reasons outlined by your professor.