The Alloy of Law

This book is a short introduction (well, relatively short, by Sanderson's standards) to the second Mistborn era. I can't think of another series I've read like this one where the first part becomes the lore and religious background of the second part. The setting and style of these books is totally different but still immensely entertaining. There is continuity with the magic system, but it turns out there is still plenty more to it than we thought we knew.