I haven’t read the book, but my wife tells me that the first two acts of this film (the ones I call “maddeningly intriguing”) are the closest to the source. It all comes together in the end, but the re-emergence of Finn’s benefactor took the film on a seemingly unnecessary action-adventure bent.
That said, most of the film is still maddeningly intriguing, and in a good way. I love the way an enormously long shot in the middle of the film (even though it’s actually composed of four shots in a nearly seamless blend) draws you into Finn’s blind panic. Good show!