Some of the popular child's books have tough words.  The author is trying to keep with a certain rhythm and rhyme and pick very advanced words.  Don't worry.  Go to google and search for the title and author of the book.  Many times someone has a youtube video of them reading the book or a famous person has read the book.  This is a great way to get the rhyme pattern of books like "Chica Chica Boom Boom" or the words of "Where the Wild Things are."  The internet can be our friend.