My favorite book, as a preteen. ( And now again as an adult.)
The gradual bonding, & shared adventures which build towards an unforeseen divergence between them, is heartbreaking & rewarding at the same time.
The tales told by the old Phoenix, to the young boy, turn out to have very different outcomes, when the Phoenix agrees to take David to visit the characters.
Neither know that there is a preordained finality to their relationship.