The problem is that several of the books are very, very specific. Sharn: City of Towers, the Secrets of, and Stormreach are all single locations (or at least, single areas, like Xen'Drik/Sarlona). So unless you're going to make your campaign heavy in Sharn, or heavy in Stormreach/Xen'Drik, then those books aren't going to be as useful. Dragons has few material your PCs are likely to come into contact with.
That said, Sharn is a very well put together book (albeit lacking in an index; you have to go to Keith Baker's website to get it). I still really dug the Stromreach/Xen'Drik books more than Sharn, but that's just me.
For my money, I'd say Dragonmarked and Faiths of Eberron. Religion and the Houses are so significant to the setting, and they're everywhere you go, so you'll get the most utility from them. Not to mention I thought the Dragonmarked book was very good.
However, I do agree with the above that the Dragonshards are an invaluable resource. And the best non-Sourcebook for Eberron is the Vicktor Saint Demain adventures in Dragon; if your campaign is set in Sharn, there's no reason you should not use those adventures.
That said, Sharn is a very well put together book (albeit lacking in an index; you have to go to Keith Baker's website to get it). I still really dug the Stromreach/Xen'Drik books more than Sharn, but that's just me.
For my money, I'd say Dragonmarked and Faiths of Eberron. Religion and the Houses are so significant to the setting, and they're everywhere you go, so you'll get the most utility from them. Not to mention I thought the Dragonmarked book was very good.
However, I do agree with the above that the Dragonshards are an invaluable resource. And the best non-Sourcebook for Eberron is the Vicktor Saint Demain adventures in Dragon; if your campaign is set in Sharn, there's no reason you should not use those adventures.