If you are familiar with Bible, the book really helps with the geography, how Israel was governed, Herod and the three year ministry.
For example, Jesus overturned the money tables on his first and third passover visit. The book goes into detail to this detail which helps clarify his three year ministry. Yet the movie only showed the last incident.
Also the historical inaccuracies in the movie as mentioned in this thread are NOT in the book. The very inaccurate exchange between John the Baptist and Jesus is not in the book.
The movie leaves the impression that Jesus come to realize he is the Messiah rather than the biblical account that Jesus was the incarnate word. The book does is not imply Jesus comes to understand he is the Messiah, but supports he claimed to be the Messiah.
Thanks - I have not read the book, so this helps. Movies very often miss the point of the book.