Can be bought from the following merchants:Bodahn Feddic. His inventory resets after the first visit to the camp, and he sells another one before the final battle, so it's possible to buy up to three copies from him. Garin
This is one of the duplicable tomes, along with the tomes for spells, skills and stats. If you intend to take advantage of this exploit, make sure you keep (don't use) at least 2 of the available Tomes of this kind, either at the original merchant or in your inventory. It's particularly easy to forget about that in this one, since there are only two copies at vendors at any given moment, and except for Bodahn's inventory reset, those are the only ones in the game.