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 describing the acts of Jehoshaphat is mentioned in a book called "the book of Jehu" which is itself mentioned in another book, "the book of the Kings of Israel."