Sharn: CoT is a must buy.
For fluff, I'd recommend Faiths of Eberron (does an excellent job of fleshing out religion in the campaign setting, especially the Sovereign Host) and Five Nations (while some might consider some of its contents a retread of material from the ECS, I think it's worth it for the 'Five things that everyone from X knows' sidebars alone. Also, while not fluff, I love the Bone Knight and Phantom Dragoon PrCs). Also also, read the hell out of the Dragonshard web articles; they are fantastic.
I'd recommend fluff before crunch, but if you're looking for a bit of the latter after you're sufficiently fluffed out, I'd recommend Dragonmarked; it has a good amount of fluff, but the extra options given that the feats give to 'marked characters are superb. I'm particularly fond of the added material regarding Aberrant marks. The crunch in Races of Eberron is also great, but aside from the magnificent Kalashtar chapter, the fluff ranges from mediocre (Warforged, Shifters) to terrible (Changelings - just ignore that entire chapter).
Novel-wise, I don't read much gaming fiction as a rule, but I really enjoyed the Dreaming Dark trilogy, particularly the first book (which has an excellent glossary in the back). I'm tempted to look into the Binding Stone trilogy based on the consistently strong accolades it has received here and other places.
You may also want to check out the Eberron forum on the WotC boards; while the signal to noise ratio fluctuates pretty wildly, Keith Baker posts there quite frequently - it's a terrific resource to have available!