Brazie’s Black Book of Secrets

Brazie's Black Book of Secrets is a consumable item which "Contains some of the best kept secrets in Azeroth. Not for the faint of heart!".

It costs a whopping 22,000 gold and contains 5 books:

The NPC who sells these items is Brazie Getz and he spawns in Deathbringer’s Rise after the roleplay, after you defeat Deathbringer Saurfang in Icecrown Citadel as an Alliance player.

And yes, he’s level 69. Quite a gnome.