I've also had the same exact issue, I turned down that quest so am unable to get the Tenebrium skill. Additionally I got in a, err... small fight with the fine people of Sacred Stone and the skill book isn't on top of the shelf that i've seen in other peoples screenshots.
Don't get me wrong, this is a fantastic game but this has basically broken it for me =(
Pleeeeease tell me theres a fix in the works and coming soon =/
