Update 4 April 2013: I get lots of questions about the subjunctive and how to use it and when to use it. It's a complex aspect of Italian grammar, but this book really makes it a lot easier to use and to understand. Check it out!
It's not often that a book on Italian grammar sells so well in Italian bookstores, but this book, Viva il congiuntivo : come e quando usarlo senza sbagliare, has been selling well on several book indexes, at least within its category. This is because the subjunctive is also complicated and difficult even for Italians!
I recommend this book for two reasons:
1) It's simply very well-written, and it presents the subjunctive and its uses in a very well organized and easy to follow manner.
2) It clears up a lot of the confusion on what the subjunctive is and how to use it -- after reading this book, I'm sure that it will dispel any confusion you may have.
People have scoffed at my recommending a book on Italian grammar for those learning Italian. I know that a resource in English that explained the language (in translation) might be easy to grasp for English speakers, but I would argue that this book is so well-written that even a student with advanced beginner skills could follow this. While the prose sections of the book are interesting, "the meat and potatoes" of the book are the chapters on the rules and usage -- from pages 71 onward. Here the authors, Valeria della Valle and Giuseppe Patota (both of whom also wrote Il nuovo salvalingua -- read my review of that book here), present a common sense, no nonsense appraoch to using the subjunctive with excellent examples.
If you're not sure of how to use the subjunctive, you have to read this book. English texts that explain Italian grammar do poorly at explaining this aspect of Italian grammar, but with this wonderful resource, all your questions and confusion will disappear!