Paper and Fire, by Rachel Caine

Jess Brightwell and his friends have been absorbed into the fabric of the Great Library, with assignments that suit their various talents, except for Jess who has been marked as a troublemaker. He has discovered that their brilliant engineer Thomas, thought to have been executed, is actually being held prisoner. The team plans an intrepid rescue, becoming outlaws in the process. All sorts of family loyalties are tested to breaking point as the embattled young people realise that revolution requires sacrifice. Suspension of disbelief becomes increasingly difficult with many inexplicable and terribly convenient plot twists. But the relentless action pushes through to a bitter end, where unexpected and uncomfortable allies are found.