While I'm a fan of Mrs. Peregrine and all her children, I found book 3 to be lacking just a bit. There were too little of the parts that made the first two books special (peculiar children, for example), and too much of the stuff that made the first two books odd (the story written to match vintage photographs instead of the vintage photographs used to compliment the story, for example).
Book 3 was full of those little annoyances that sink otherwise good books--loose strings that don't get resolved, characters of significance written off to unsatisfactory endings, and a sluggish plot that don't got to the meat until the last 15% of the book--to name a few.
I was very pleased with the series conclusion, however, and that is the saving grace of Library of Souls.
Fans of the series are going to read the last book in the trilogy no matter what. If you haven't picked up book one yet, I say go for it. There is enough fun to be had that make this peculiar journey worth the ride.