As much as I liked Libriomancer and loved its world, Codex Born and I didn't get a good start. The reason is in the multiple first person voices. What I expect from a second book in a series is to learn more about the characters and their relationships interspaced the book specific plot, but I also hate headhopping especially when the voices are indistinguishable. So, when a story is told from a certain point of view in a first person voice, I expect the author to stick to it.
One benefit from Lena's chapter inserts is the pacing, which is much better here than it was in Libriomancer. There really isn't much breathing room between the action packed scenes, which in theory should have sucked me in without an escape.
The story itself is an organic continuation to the events of the first book. The world becomes more complex and morally grey as do the characters. Some new secondary characters are introduced and old ones sidelined. Also cultural diversity expands. Yes that is code for diverse secondary characters who are crucial to the plot.
I can't say much more than that without spoiling either Codex Born or Libriomancer. And to be honest: I don't remember much more.
There was even a slightly dated but accurately explained Finnish curse word. I myself prefer to invoke the devil rather than a deity.