Tim Pratt, one of my favorite authors, has just announced that his novel The Nex will be free to download (Amazon US, Amazon ES) for the next few days. This is the product description:
Teenager Miranda Candle finds a mysterious necklace and is suddenly transported to The Nex, the bizarre city at the center of all possible universes, where she falls in with a pair of would-be revolutionaries -- the skinshifter Howlaa and the bodiless Wisp -- fighting the oppressive regime of the city-state's Regent and his army of steam colossi, cynical cyborgs, and the depraved royal orphans.
This is a wonderful YA novel, one of the best I've ever read, and I can't recommend it highly enough (in fact, I made a donation while The Nex was being serialized online and my name is on the acknowledgements of the novel). If you haven't read Pratt's work before, give it a try. I'm sure you'll love it (and the go on and read his Marla Mason novels and his story "Impossible Dreams": they're all amazing).