So it's been over a month since I finished this, and the details have largely slipped my mind now, but I do remember it being a fast, fun read. I apologize in advance for this being one of my more vague reviews -- I really need to write them right after I finish if I want to be thorough.
Codex Born is the second book in Jim C. Hines' Magic Ex Libris series, which follows libriomancer Isaac Vainio, who can do magic by tapping into the collective unconscious of the reading public and pull things out of books. Isaac works for Die Zwelf Portenære (or The Porters), a secret society of libriomancers founded by Johannes Gutenberg hundreds of years before. Only, Gutenberg is still alive, having used the magic practices he founded to extend his lifespan (possibly making himself immortal in the process) and Isaac is only starting to learn that there are many many secrets Gutenberg has been keeping from those who follow him.
The plot of Codex Born largely revolves around Isaac and a group of his friends attempting to track and eliminate a new threat to their magical world that has popped up in the form of their dead colleague Victor's mentally unbalanced and rather power hungry father, who has gotten ahold of some of Victor's magical inventions posthumously and is now in waaay over his head. It starts out simply with some murdered wendigos, but soon Isaac realizes Victor's father has accidentally put himself in the middle of several factions of unknown magic, including a secret society supposedly wiped out rather violently by Gutenberg hundreds of years before, and an unknown magical threat that seems to be coming from the books themselves.
I very much enjoyed this book. There were parts to do with the central mystery of the series that were a bit too nebulous (and thus confusing for me), but I liked Victor's father as the villain, and I liked that the real threat was only awoken on accident. Hines's characters are a lot of fun, as is the magic system he has developed (although I did find myself wishing, with this being the second book and probably not very many to come after this, for a bit more backstory and worldbuilding -- these books are just too short). I also enjoyed the development of Lena in this one, although instead of her diary entries at the beginning of each chapter, I think I would have preferred the book to just be from her POV (although I can understand wanting to keep it Isaac's story).
There's nothing really deeply profound about this series, but who made that a requirement, anyway? What it is is a heck of a lot fun, and I'm excited to read the remaining book(s) when they come out.