Sword in the Stacks — Jen Swann Downey

As the newest apprentices to Petrarch's Library, Dorrie and her brother are determined to help search for the missing key that could destroy their beloved Library. On a training mission to England 1912, Dorrie sees her chance to help. All she needs to do is get close to a lord with a connection to the Lybrarians' enemy - the Stronghold. If Dorrie can earn the lord's trust, she might be able to get the Lybrariad just what they've been searching for. But if she arouses his suspicion, she could lead the Strongholders right to Petrarch's Library... and everyone she cares about.