Traveler's guide to the Shadowfell:
In this book we follow a caravan of Vistani (which is now a Background you can pick, just like it was a template in 4e; any race could be Vistani, since they welcome any lost traveler in the Mist willing to learn their customs) through the Shadowfell, where the demi-planes/prisons are located. We see Gloomwrought and the Palace of memories of the Raven Queen and a brief overview of the different prisons of the darklords and tables for the Dark Power effects in the region.
We also receive nice tables to make the dark-mirror version of your cities on the prime material plane, with Neverwinter/Evernight as an example.
The second book is X guide to Innistrad, where we pick off where we left our fellow Vistanis and now we use the mist to visit this MTG plane, with expanded sanity rules, corrupted monster template etc