You return torn pages by visiting the book in Merlin's house. (If you cannot get to Merlin's go to the 3rd district in Traverse Town and look for the door with the fire symbol. Cast fire on the door and it will allow you to open the door. Cross the "lake" and into the house. After a cut-scene and some introductions, look at the book on the nightstand. If the book is not there, return to Cid and return to the book to Merlin. He will explain that the book is missing the pages and then you can recollect the pages and insert them. This all takes place when you return to seal the keyhole in Traverse Town.) Simply put, all you must do is find the pages (one by one or all at one time), and simply visit the world within the book. But, some times the page will stay in your Items section, to remove