The appositive in the given sentence: J.M. Barrie's book Peter Pan was made into a play and a movie, is (a) J.M. Barrie's book. Appositives are noun or noun phrases which gives another name to the noun near it. They are very easy to trace. In this case, Peter Pan is a noun. It is renamed into J.M. Barrie's book. Thus, "Barrie's book" is an appositive of Peter Pan.
Additional Notes:
(b) was made: is the verb
(c) Peter Pan: is the main noun
(d) into a movie: is the prepositional phrase