Gooseberry Bluff Community College of Magic: the Thirteenth Rib by David J. Schwartz is the first Kindle Serial I've tried. Serials are one time purchases episodically delivered as the author completes shorter installments.
Kindle Serials hold the hope of performing like an old time radio show. I enjoy looking forward to them.
Very much in the vein of the Magicians by Lev Grossman, Gooseberry Bluff is set in a future where magic is real and taught. The first episode hooked me as a Federal Bureau of Magic agent is sent undercover to investigate a community college professor's disappearance, apparently related to a larger demonic summoning/terrorist plot.
Schwartz uses 30-40 page installments to develop the story very well. I hope this format catches on.