The Sepher Yetzirah is a short treatise, probably done between 2nd and 5th century AD. There are different versions, the longest has 2000 words.
It is not kaballistic, if you define Kabbala as being started around 1170 AD, but there were more than 50 commentaries in medieval time already.
The essential text is rather simple, it was even suspected, that it was a schoolbook for children, just mixing some things together, for instance Hebrew letters and astrological contents, which became the main content and as such the Sepher Yetzirah influenced deeply the Tarot interpretation from Eliphas Levi on, as the astrological row of Sepher Yetzirah (or one of its rows, as there are different versions) was projected upon the given row of Tarotcards. So it happened (for instance), that the medieval Tarot-Moon (clearly meant as a Moon and nothing else) became suddenly the astrological sign of Pisces.
So understanding the Sepher Yetzirah as the real source might lead you to understand the Tarot-development of 19th century (Eliphas Levi and the French school and the alternative interpretation of the Golden Dawn).
A very IMPORTANT book, let's say the key to it all.
But probably only, if you read the Sepher Yetzirah and not its many explanations
