Angels and Demons was a Prequel. The movie studio's made it a sequel 

The book was fast paced. It was like National Treasure or Jurassic Park. It just sped along so smoothly. The book was able to paint a fascinating story the readers could easily imagine. The twists and turns and moving forward just was so easy to digest. If I recall, the movie just got bogged down in all kinds of crap, disrupted the flow, etc. The movie did not reflect the book very well.