The book is described in : "Now the rest of the acts of Jehoshaphat, first and last, behold, they are written in the book of Jehu the son of Hanani, which is mentioned in the book of the Kings of Israel."
The book detailing the actions of Jehoshaphat, both beginning and end, is found within the book of Jehu, the son of Hanani. This book is also referenced in the book of the Kings of Israel.