A few that come quickly to mind (sorry for the obvious Eberron-fanboy-ism)
Eberron: Nine Nations (opposed to Five Nations) - detailing the Demon Wastes, Shadow Marches, Drooam, Zilargo, Mror Holds, Valenar, Talentia, Q'barra, and the Principalities.
Eberron: Secrets of the Dhakanni - detailing the old empire and the new rising nation
Eberron: Secrets of Noir - (Written by Keith Baker and Nick Logue, of course) Detailing concepts of noir, how to have anti-heroes, and running intrigue/noir campaigns
Eberron: Secrets of Khyber - As mentioned above, a book giving ideas on the ecology and geography of the Eberron underworld.
Eberron: Beyond the Veil - a sourcebook on the planes of Eberron.
Eberron: Nine Nations (opposed to Five Nations) - detailing the Demon Wastes, Shadow Marches, Drooam, Zilargo, Mror Holds, Valenar, Talentia, Q'barra, and the Principalities.
Eberron: Secrets of the Dhakanni - detailing the old empire and the new rising nation
Eberron: Secrets of Noir - (Written by Keith Baker and Nick Logue, of course) Detailing concepts of noir, how to have anti-heroes, and running intrigue/noir campaigns
Eberron: Secrets of Khyber - As mentioned above, a book giving ideas on the ecology and geography of the Eberron underworld.
Eberron: Beyond the Veil - a sourcebook on the planes of Eberron.