Joseph Dan presents a phenomenal, scholarly, introduction to the Kabbalah as a philosophy and a phenomenon. This book doesn't go into the theology or the specifics of the various schools of Kabbalistic thought. Rather,the author provides an overview of the history of Kabbalah from the beginnings to modern day.

If you are looking for a book that is written by a Harvard professor - this is it (and the author is one). If you are looking to do "pathworking", or study of the Tree of Life, or read the Zohar - look elsewhere.