Answer: There are more ancient books than The Book of Zohar, but it is unique because it describes a person’s whole spiritual path, his continuous ascent, the correction of his soul. The other books are not meant for the continuous progress of the one who attains. This is the reason that The Book of Zohar is unique and special for us.
The main thing is that it was written from the top of the ladder of attainment. The people who wrote it were the greatest Kabbalists.
From the Kabbalah Lesson in Russian 2/5/17