Some meta-spoilers ahead.
This has been my favorite out of the first five books (maybe comparable to the first one). After a very slow beginning, we get to read see perspectives of the four main characters. They all split up and handle some things related to their back-stories (expect for Holden who is mostly just bored from being all alone). When I was of the impression that this might be the most boring book ever, the biggest crisis (up to this point in the series; I haven't read the newer books yet) takes place and the different storylines slowly come together again - leading to a thrilling and suspenseful conclusion.