Obviously, when you read a book you get to direct the movie in your head. All the details are perfect, unless the book really sucks.
You're right about putting a book into a 2 hour format. A perfect example is the recent release of "Soldier, sailor, Tinker, Spy". The movie was good, but if you hadn't read the book it could be confusing and can't compare to the excellent BBC miniseries from years back.
