This Book of Rituals belongs to the mage Vithal but is in the possession of an Innkeeper in the Underdark. Vithal asks you to return it to him as a part of the quest A Mage's Proposal.
To obtain this tome, you can either pay the Innkeeper 300 
or approach him while under Adalon's Drow-form illusion, in which case he will give you the book for free.
Description[]
| “ | This is a book of elemental rituals that is the possession of a mage called Vithal. It is of little use to any other person. | ” |