The Tree of the Sephiroth, from J.D. Mylius, Anatomia auri  (Public Domain Image)
by Bernhard Pick
This is a short monograph on the Kabbalah, written from a purely academic and somewhat critical point of view. The literature about the Kabbalah in the public domain in English is very limited. Most of the key texts and scholarly studies are in Hebrew, German, and other languages. So this well-crafted review of the literature is a real find.
Pick traces the origins and background of the Kabbalah to Jewish scholars of mediaeval Spain. He does not attribute any deeper roots to it historically, nor does he examine the roots of Jewish mysticism except for a brief mention of the Merkabah. Pick represents the Kabbalah as a backlash to the rationalism of Maimonides. He also discusses it as if it were purely of historical interest, ignoring the ongoing and lively popular interest in this belief system even in his day.
Pick provides a dissenting view to much of the uncritical literature produced at the turn of the 20th century by more mystically inclined authors, and as such is worth reading to get a more rounded view of the subject. If you are looking for an introduction to the subject written by an advocate of the Kabbalah, it might be better to start with some of the other books on the subject available at this site. But if you need a starting point for further academic study, this is a good place to begin.