Books: Dungeon Delve
In normal D&D games, the Dungeon Master isn't an enemy of the players. Instead, the DM is the person who runs and guides the adventure, tuning the difficulty to give the PCs a good challenge but not an overwhelming one. After all, the DM has absolute power and could make the monsters in the adventure arbitrarily difficult, so it doesn't make sense for him or her to "play against" the player-characters. The recent D&D supplement "Dungeon Delve" takes a different approach, though.
Here, D&D has become a competitive game, with the DM actively trying to hinder or kill the characters (within the rules, of course). "Dungeon Delve" contains 30 challenging minidungeons, each filled with three encounters. The final encounter of each minidungeon typically features a very difficult fight designed to push the PCs to the limits. Some encounters have such overwhelming odds that only the most optimized parties have a chance at survival.
For instance, there's an encounter designed for Level 3 characters that contains a Level 8 monster. Now, it's hard for a typical Level 3 character to even hit a Level 8 monster, let alone deal enough damage to whittle down its 200+ hitpoints. Even the normal D&D rules caution against using monsters that are 5 or more levels above the PCs. But "Dungeon Delve" isn't normal D&D, it's D&D as a tactical boardgame.
Each minidungeon in "Dungeon Delve" can be built entirely from dungeon tiles that Wizards of the Coast has released, making the whole affair feel rather cut-and-paste. These minidungeons are designed to be either run through in a single night or combined with other encounters or ideas. There are some new monsters in "Dungeon Delve," too, but its bestiary isn't overflowing like "Open Grave" was.
So is this worth the money? I suppose if you need a quick encounter, "Dungeon Delve" provides a quick and easy source that covers literally every level of D&D gameplay, all in a nice hardcover package. Just be careful that you don't throw in too many of those finale encounters, assuming you want your player-characters to survive.