Five Nations
If Xen'Drik is the adventure continent, the Five Nations of Khorvaire are Eberron's centers of intrigue. There is a lot of potential here for covert campaigns and sharper role-playing then the hack and slash typically associated with D&D.
Seeing as how much of Khorvaire was gone into with the Eberron Campaign Setting I can feel a little cheated after reading this supplement. 'Five Nations' covers some of the same ground as the core book, but only so it can properly expand the NPCs, the workings of the government, its institutions, and many more wheels turning in the background that could not have found space in the campaign setting before. I love the glut of information these books provide. Its difficult for me to imagine my friends getting so involved in a campaign that they would debate cuisine or wine in character, but its nice to know its there.
All major players, some only referenced by name and level before, are given full stats and equipment, and there are multiple new locations and adventure start-ups included. A very nice resource.