Clue: Convert you mentored, producing book from OT (11)
Answer: DEUTERONOMY
Explanation: Definition: book from OT
theme: Old Deuteronomy is the name of a character in *Old Possum's Book of Practical Cats* and the musical adaptation *Cats* 
definition: Deuteronomy is the fifth book of the Old Testament ("OT")
 
anagram/"Convert" of (you mentored)