The first in this series is Relic (reviewed back in February 2013), which I liked. Reliquary takes the Museum Beast from the first book and goes deep underground to the tunnels of Manhattan. The sequel held my attention and was ok but definitely not as good as the first book.
We meet up with all the regulars from the first book: Margo, D'Agosta, Pendergast, Frock and Smithback. A few new faces are thrown in but don't bother with some of the names, their heads come off pretty quick. Something is out there killing folks again and taking the heads of the victims. No one really cares when the homeless are being preyed on but when a debutante loses her head, well, now we need to do something about this!
The authors claim that the Astor Tunnels (aka Devil's Attic) are real. A google search comes up inconclusive (but does point to this book as a source). Supposedly, thousands of homeless live in the abandoned tunnel system under NYC and since the Museum Beast abhors light, he goes underground. He...she....all of them. Because now there are many, many Museum Beasts.
Kawakita, a scientist from the first book, thinks it's a good idea to isolate the virus that manifests people into Museum Beast and sell it as a drug. I don't even know what to say about this one.
The ending, with the leader of the beasts unveiled and all the chaos that happens, was, at best, eh. It was a twist that wasn't completely unexpected.
I would say, this is a readable book, but stick with the first one and just enjoy that.