You must find all the torn pages in order to complete 100 Acre Wood. This quest becomes available once you bring the book to Merlin in Traverse Town. Here are the locations of all the pages.
01. Agrabah - Cave of Wonders - Dark Chamber (in a chest)
02. Monstro - Chamber 6 (in a chest)
03. Atlantica - Ariel's Grotto - Examine the shelf (in a chest)
04. Halloween Town - Research Lab - Examine the bookshelf
05. Traverse Town - Dalmatian's House - Reward after finding 51 puppies.