Handbook of Backstab IX teaches Backstab (Rank 9).
Handbook of Backstab IX drops from any of the bosses in the Ruins of Ahn'Qiraj.
As of Patch 2.0, Backstab (rank 9) can be learned from any rogue trainer at level 60.
Patches and hotfixes
Patch 1.9.0 (03-Jan-2006): Added