Films inspired by books seldom include EVERYTHING that is in the book.
The movie would simply become too long.
If you have never seen The Princess Bride movie, be aware of spoilers in this great Cinefix video that points out many differences between the book and the 1987 movie.
The Princess Bride
Read more at Wiki.
Consider how your current project would translate to the big screen.
What would be cut?
Here is a list of books that have been made into movies.
Of course, one that we are eagerly waiting for is:
The Scorpio Races.