The film starts with Shrek reading the book. The book says that Princess Fiona "had an enchantment upon her of a fearful sort" and that she was "locked away in the highest room of the tallest tower". Later, Shrek tells Donkey to look for some stairs (in the Dragon's Keep rescuing Fiona) and that Fiona will be at the top of the stairs in the highest room of the tallest tower. Donkey asks how he knew this and Shrek says that he read it in a book once. But surely if he remembered what the book said he's also know about the enchantment, especially when Fiona kept talking about her "true love's first kiss", which was also in the book that Shrek read in the beginning.