When I saw it, I didn't know it was based on a book. I agree with you that a two-hour movie just can't do an entire story justice; there's so much more to tell. It seems like they do that on purpose as a marketing strategy to sell more books, but that's just my own speculation. Since I didn't know it came from a book, I thought the concept was a really bold move—an intelligence agent who wasn't a super soldier who fights everyone and kills them without a scratch, but someone who used their mind instead of brute force.