There is no book. When you get to the Outlands and fly to your respective faction starting town (Honor Hold or Thrallmar), find a the tower on a hill (yes, both towns have a tower on a hill) and speak to the Herbalism trainer inside. He'll be next to the Grand Master Alchemist trainer.
Or you could speak to any of the Herbalism trainers you've come across before. The primary gathering professions can be uncapped by any of them.
Edited, Feb 12th 2007 8:22am by thermalnoise