@Bookworm You've given a tough task with this one. Obviously the start and end of the book is written to make sure the journey to Fantastica is
tvtropes.org/pmwiki/pmwiki.php/Main/MaybeMagicMaybeMundane . There's no definite proof remaining of it, Bastian doesn't bring anything back from Fantastica and doesn't get any superhuman powers, but at the same time it's hard to explain how he changed so much if it was just a dream and there was no book at all.