Book 4 in "The Invisible Library" series. 

When Dragons pull a Librarian into their politics, Irene is sent to a 1920s America to find the Librarian in question and find out how much trouble he was in with the Dragons and bring him back to The Library to answer for his breach. Irene and Kai face gangsters, blackmail, and Noble Dragons trying to persuade her to take their side. 

I enjoyed how the storyline unfolded and the speed in which it did. Once it took off, it was like rolling down the first hill of a rollercoaster and continued at that pace with momentary slowdowns in a cycle until the very end. All of the books so far in this series have been paced as such.