The Book itself was also very short. I didn't feel like very much happened.
From what I have seen most people reported around 4 to 5 hrs play time. Compared to Books 1 & 2 it is short. If I remember rightly, the entire game was originally estimated for a playing time around 10 hrs. Also, they intended it to publish the complete game, instead of books. If you look at all three books together, instead of separately, we have been given a game much larger than what they had originally planned. Again total play time for many people is close to 20 hrs and we still have 2 books to go. Ragnar said those books will be around the same length as Book 3, but that still adds another 8 to 10 hrs and a total of 30 hrs for the game. About triple the original estimate. It took me 57 hrs to play Dishonored and that was a AAA game and much more expensive to buy when it first came out than DFC.
For me and others it seemed like quite a bit happened.
By the way, I thoroughly enjoyed the picture puzzle that many found annoying.