Codex Born by Jim C. Hines
Isaac, Lena and Nidhi are back in Codex Born the second book of the Magic Ex Libris series by Jim C. Hines. With the murder of a wendigo, a mysterious and powerful organization makes a reappearance after over 500 years of silence with an agenda to use Lena Greenwood (a dryad) as a weapon against the Porters.
Die Zwelf Portenaere (Porters) were created by Johannes Gutenburg to protect the world from supernatural threats and control the libriomancers, who use magic to reach into books and pull out the desired object. While this book is action packed and a continuation of concepts, events and relationships in the first book, it is also more Lena's story, her struggle to find her strength and her individuality.
Much of the set up for this book is laid out in the first of the series Libriomancer. If you are new to the series you may want to start there to get a grasp of all the fun sci-fi concepts presented there, as well as the intro to the complex relationships between the characters.