Best books for learning data mining

I'm fairly new to data mining myself, but I've been reading a book called "Data Mining: Practical Machine Learning Tools and Techniques" by Witten and Frank. The book is a little light on theory, but it has a pretty good overview of many different techniques for data mining, and it has enough detail that you can start applying some of the techniques on your data immediately. I'm using the book in conjunction with a course on decision trees that I'm taking at statistics.com, which I would recommend. "Elements of Statistical Learning" seems to be the closest thing to an authoritative text on data mining that I've found, and it has more theory than the book I'm using for my class, so that may be of use to you. I'm planning on studying it more after I've finished taking my class.