I read about 5 of the Princess diaries books, before i stopped. I was amazed to see that there are 10 books out now. There have also been two movies made based off of the acclaimed series.
In the books we get to experience more of Mia's day to day life and see that even after she becomes a Princess that she has the real world to deal with and sometimes it just is not pretty. As far into the books as I got The grandmother was a scarier woman than in the movie. Mia did not really like her, and she was portrayed as strict and not as loving as the character portrayed by Julie Andrews. I believe that in the last book I read Michael was playing an active part in Mia's life, but in the latest movie he was completely gone. While I did love Princess Diaries 2 it was nothing like any of the books I read. I think that after seeing the first movie, people needed an ending and thus the movie was made. I believe that the largest difference between the books and first movie was the portrayl of the grandmother. In the books there as a woman who I pictured to look more like Cruella de Ville, who did not have a whole lot of love to give. In the movie there was Julie Andrews, enough said. I do not think one is better than the other but there are advantages to reading the book as well as watching the movie.