Just to change things up a bit ...
Why this book now? Because it's one of the two books I never picked up for 4E and I figured I might as well finish the set.
This is subtitled the "Dungeon Survival Handbook" but it's a mix of material for players and for DMs. It is very much optional and I never missed it while I was running my 4E games.
- Up front is a selection of Themes for a dungeon-heavy campaign. These were a late addition to 4E that added a lot of flavor and a little bit of crunch, first appearing in Neverwinter I believe. I liked the concept and I still do. There are 7 new ones here and they seem broad enough to be useful in a lot of campaigns if a little less flavorful than the Neverwinter themes. "Treasure Hunter" and "Trapsmith" are samples of what we're talking about here.
- We also get new races: Goblin, Kobold, and Svirfneblin. Very underdark-themed, but I could see them being fine in some games.
- There is a section of dungeon-themed alternate powers for a bunch of classes. Nothing essential, and nothing I saw seemed to be a game-breaker, but some cool moves nonetheless.
- That's about it for the mechanics other than a few pages of magic items near the end. Most of the rest of it is good ideas for exploring dungeons, types of dungeons, things that live in dungeons (no stats, just flavor, habits, descriptions), famous dungeons (White Plume Mountain, etc) , DMing dungeon adventures, and methods for creating dungeons including a random dungeon generator.