I recall when I first became interested in Kabbalah in 1969.  There were very few books on the topic available at that time: a couple by Gershom Scholem, a few on Hasidic legend by Martin Buber (some of which touched on Kabbalah) and some others of varying reliability.  Nearly 40 years later, the very opposite problem exists ... there are so many books on the subject, where does one begin?  The following information is intended to help you choose.

Firstly, the spelling of the term varies:  Kabbalah, Qabala, Kabala, etc.  All refer to the same Hebrew word which simply means "reception", in this case reception of an esoteric spiritual tradition which goes back to antiquity (there is considerable controversy as to exactly how far back).  Secondly, there are several different branches on the Kabbalistic tree, and accordingly several sorts of book which give different types of information.  Knowing how to distinguish these is an essential first step.

One type of book is the scholarly/historical.  These works trace the background of kabbalistic concepts, examine texts comparatively, and/or discuss the development of mystical ideas and philosophies.  These can be somewhat dry, but provide a treasure trove of factual information.  If this is what you're looking for, you can do no better than to grab a copy of one of Gershom Scholem's masterful books.  This Israeli scholar (1897-1982) devoted his life to enhancing respect for esotericism and mysticism in Jewish historical research.  "Major Trends in Jewish Mysticism", though first published in 1943, is still a classic and is fairly easy to read.  Another good first pick would be Scholem's later work simply titled "Kabbalah", which consists of his Encyclopedia Judaica articles from the 1960s.  If you want to examine some later scholarship (some of which takes issue with Scholem's theories), you could check out works by Moshe Idel, Rivka Schatz-Uffenheimer, and Joseph Dan.

Another sort of work is the occult/hermetic.  These ultimately derive from Renaissance scholarship carried out by Christian seekers.  Though based on Judaic roots, the work of Pico della Mirandola, Johannes Reuchlin or Cornelius Agrippa had quite different emphases and added wisdom from Ancient Greek and Egyptian sources as well.  My favorite modern exponent of this tradition is Dion Fortune (aka Violet Mary Firth).  Most of her books are currently in print and she is a very clear and sparkling writer.  "The Mystical Qabalah," in its current (corrected) edition, is particularly recommended.  Arthur Waite and MacGregor Mathers are two other important Occult/Kabbalah masters, though their books are somewhat old-fashioned in approach and style.  Lon Milo Duquette is a younger writer with a great sense of humor; his "Chicken Qabalah of Rabbi Lamed ben Clifford" is great fun and quite informative too.  The emphasis in this type of book is less on scholarship and more on practice - meditation and ritual.

Probably the most important type of Kabbalah book consists of the classic texts themselves, and several good English translations have appeared in the last few decades.  Works in this category are the best for inspiration, creating a contemplative mood and inspiring the reader.  Perhaps the best place to start would be with the beautiful volume of selected readings entitled "The Essential Kabbalah: The Heart of Jewish Mysticism".  This will give you samples of the many flavors and moods of Kabbalistic literature.  For individual texts, the most revered and beloved Kabbalah classic is definitely the "Zohar" (or Book of Splendor) which first appeared in the 14th century and was written in Aramaic.  Again, there is controversy as to the work's ultimate origins.  A multi-volume complete translation by Daniel Chaim Matt is in progress, but this will probably be too advanced (and expensive) for beginners.  However, the Paulist Press has published an excellent one-volume abridged version also translated by Matt, with his masterful commentaries.  Scholem (see above) did an even shorter selection in the 1960s; this is published by Schocken books.  Another basic text is the "Sefer Yetzirah" or "Book of Creation"; this is much shorter than the Zohar and many translations are available.  I recommend the one by Aryeh Kaplan (published by Weiser) which has great commentaries. 

Something must be said about the best-marketed approach to Kabbalah in our time, that of the Kabbalah Centre headed by Rabbi Philip Berg.  Books by him, his sons Yehuda and Michael, and wife Karen are many and quite well-distributed.  The Centre has introduced Kabbalah to an amazing variety of people, including Demi Moore, Madonna, and other media stars.  This approach is quite distinctive as it downplays traditional approaches in favor of a quasi-psychological, New Age emphasis.  I would recommend "Education of a Kabbalist" by Rav Berg because it shows the roots of his teaching philosophy.

I have merely touched here on a few of the many books currently available on Kabbalah.  You must now choose which approach best suits you.  May you find wisdom and blessings all along your path!