Answer:
   
   The best translation and commentary on this is by Rabbi Aryeh Kaplan.
   "Sefer Yetzirah", published by Jacob Aronson. It includes the complete
   Hebrew text of all versions of this book, a clear commentary with
   generous commentary and explanation, and in depth discussions of many
   areas of Kabbalah with clear and lucid explanations and diagrams. From
   the introduction:
   
     The Sefer Yetzirah is without question the oldest and most
     mysterious of all Kabbalistic texts. The first commentaries on this
     book were written in the 10th century, and the text itself is
     quoted as early as the sixth century. So ancient is this book that
     its origins are no longer accessible to historians.
     
     Careful study indicates that it is a meditative text with magical
     overtones. Talmudic traditions indicate that it could be used to
     create living creatures, including the Golem! The Sefer Yetzirah is
     a small and concise book, only 1300 words long in the short version
     and 2500 words long in the long version. The first chapter
     discusses the Sefirot; The second chapter is a discussion of the
     letters of the Hebrew alphabet, and the 231 gates; Chapters three
     to five discuss the divisions of the letters in relation to
     astrology.
     
     The text was deliberately written in a fashion so that it would be
     meaningless to those who read it without an extensive background in
     the Tanakh (Hebrew Bible) and Midrash. To guide the novice, Rabbi
     Kaplan takes great care to introduce the necessary knowledge to the
     reader, making it accessible for the very first time to English
     speakers without a Kabbalistic background.