Eberron: What To Buy?

[Jhaelen]

Most of the Ebberon books are excellent, but with the exception of the ECS none are essential unless you plan on focusing on the same topic as the book does.

That's about the way I see it, too.
I don't have that many Eberron supplements:
I kind a liked Races of Eberron but was quite disappointed with Sharn (probably because of too high expectations after the brilliant ECS).
Secrets of Sarlona is nice as well, but doesn't provide enough depth to actually base a campaign there. It's better suited for Khorvaire campaigns that want to include short visits to Sarlona, imho.