The Skill book is somewhere towards the left or right side of this dungeon (towards the wall on a higher place if i'm correct) .
I only find this info in a earlier post above :
4: Broken Valley, Citadel Dungeon, one of the five items on chains to choose from (this is in the back of the room). (MISSABLE)

I believe at the left back side if you face the hanging weapons. Not totally sure, sorry ! But warning: don't take any of the weapons, otherwise NO skillbook ! wink

Good Luck up

On 7th of february 2015 : I start a new adventure in the Divinity world of Original Sin,
it's a Fantastic Freaking Fabulous Funny ... it's my All Time Favorite One !