Ok here's something I discovered that many may not know about (but then again some probably do). The Leaves of Essence is grimoire on level 12 of Od Nua. It belongs to a Vithrack vendor and you if steal it will cause the Vithracks to attack you.

 

It doesn't have any unique spells, but it does have a fair number of high level spells.

 

If you intend to resolve the Vithrack situation peacefully, you can actually open the drawer that contain it, right click it as if you will pick it up, and copy the spells as if the grimoire were in your inventory. Afterwards you may paste them onto another grimoire for safe keeping.

You can also take the Grimoire after resolving the Vithrak's quest without causing them to go hostile.