Prisoner of Askaban. Favorite book and movie.
I read books 1 to 4 shortly before movie 1 came out. At first I was disappointed with the first movie, but I liked it more with every time I watched it.
And when the new books were released I read all the older books before. So I read book 1 at least 4 times. Still fun. Maybe someday I will read them in english.

I added an additional page to the last book. There were several chats with J.K. Rowling where she answered fan questions of what became of some of the characters. I collected and listed them, just like some 80s movie where you see pictures of the characters just before the credits with texts of what happens to them after the movie...