I found the Explorers and Xen'drik books to be not all that useful. I don't think I've opened the Explorers book once since reading through it the first time, except perhaps to look at the lightning rail table.
Races of Eberron has lots of good, useful background for the new races, and plenty of rules options for them as well.
I thought Five Nations & Faiths of Eberron were really good in the "fluff" department. Faiths especially impressed me with all sorts of talk about variant sects of the main religions, a topic that you don't see addressed in D&D religion very often.
Player's Guide is handy if your players are into world immersion, but there's not much in there that can't be found elsewhere.
I haven't gotten Forge of War yet, nor the Magic of Eberron or Secrets of Sarlona books (and no plans on those two), so I can't comment on those.
Races of Eberron has lots of good, useful background for the new races, and plenty of rules options for them as well.
I thought Five Nations & Faiths of Eberron were really good in the "fluff" department. Faiths especially impressed me with all sorts of talk about variant sects of the main religions, a topic that you don't see addressed in D&D religion very often.
Player's Guide is handy if your players are into world immersion, but there's not much in there that can't be found elsewhere.
I haven't gotten Forge of War yet, nor the Magic of Eberron or Secrets of Sarlona books (and no plans on those two), so I can't comment on those.