Having just read Hollow City with my book club a few weeks ago, I got my hands on Library of Souls as soon as I could! Unfortunately, this was definitely my least favorite of the three. 
 This book starts immediately where Hollow City left off, and we're plunged back into the action of Jacob and Emma fleeing the wights and trying to find their captured friends/ymbrynes. Maybe it's because book 2 was almost entirely action as well, but I found myself getting tired of all the fleeing. It seems that my tolerance for young (kind of?) peculiars bouncing all over London from one bad situation to another is finite. I did start enjoying the book much more once they reached Devil's Acre and we started getting more actual plot and story progression. 
 The biggest reason that I only three starred this (and three stars isn't bad! That represents a decent book that I don't regret reading. I wish three star ratings weren't so looked down upon, because let's face it - if every book is four or five stars, the prestige of those high ratings is basically meaningless. End rant.) is that I just didn't believe a lot of the ending. 
 (view spoiler)[I didn't buy Jacob's sudden ability to control an entire horde of hollows, after struggling so hard with just one, just because they all fell asleep together. I didn't buy that Jacob wouldn't have wanted Miss Peregrine to wipe his family's memories. I didn't buy that an unanticipated result of the loop collapse was undoing the aging problem of the peculiars. And I ESPECIALLY didn't buy that perfectly-packaged happy ending where Miss P and the peculiars are now going to be crashing at Jacob's parents' house like one big happy family. In general, I don't like unrealistically happy endings, and this one seemed especially forced. 
 Also, what happened to Jacob's hollow? The one in the Panloopticon? There's a mention of Jacob asking if they can let him out, Miss P says no because they need the Panloopticon running, and then - nothing? Is the implication that that poor hollow is going to spend the rest of its life in the machine? ::shudders:: (hide spoiler)] 
 That said, I'm glad I read this trilogy, because it was fun. I wouldn't hesitate to recommend it to anyone who enjoys YAish fiction.