reading versus voyeurism: movies based on books

When I was younger I used to have read-a-long books that came with records. The majority of them were based on Disney cartoons. As I grew up I started to read books based on movies. The first book I read that a movie was based on was 101 Dalmations. I was confused because the book was not identical to the movie. My dad told me the book is always better than the movie. In my naivete I did not understand the difference between books based on movies and movies based on books.