Books: Open Grave
4th Edition Dungeons and Dragons rules supplements are usually a hit-and-miss affair. There is a delicate balance between fluff (story and background that, while helpful to DMs who need inspiration, don't actually contain new game rules) and crunch (new monsters, encounters, traps, feats, powers, classes - basically anything with hard numbers). "Open Grave," one of the latest 4E D&D splatbooks, strikes the right balance between the two areas.
"Open Grave" focuses on the undead. The beginning of the book is pretty fluffy, with descriptive text highlighting undead origins, habits, and societies. The middle of "Open Grave," though, features around a half-dozen "lairs" (basically dungeons that can be dropped right into your campaign) all themed around the undead - everything from a creepy cemetery to the skyscraper-sized hollowed-out corpse of a forgotten god. Most of these look pretty fun, and they're written to cover a wide range of player-character levels.
The third part of the book is crammed with new undead monsters - about a hundred pages worth. Again, these are nicely spread out by level - 1st level undead hounds, new types of mummies, and famous Epic-level D&D deities like Vecna. There's actually quite a bit of D&D fan service here (the monster entries for Ravenloft's Count Strahd and Tomb of Horrors' Acererak are wonderfully true to their pre-3E roots), which is a good thing.
When you look at the total package - nearly 200-odd pages of new dungeons, new monsters, and a potpourri of new game rules for $30 - this is one of the best 4E supplements that has been published.