Fullmetal Alchemist Volume 10: 10/20/11
Volume 10 of Fullmetal Alchemist resolves the Barry the Chopper story and answers the question of what happened to Maria Ross. More importantly, though, the book expands the world and the history beyond that of Amestris.
This history and world building helps to put the story so far into perspective. It also helps to explain, or rather, refine, the rules of Alchemy as we know them. So far it's been all about equivalent exchange and knowing the Truth. Now, though, there's a second type, called Rendanshu (or Alchehistory in Brotherhood).
At the heart of all of these revelations is Cselkcess, a ruined city mentioned early on in the series. Ed gets a chance to see the ruins for himself and his knowledge of advanced alchemy brings to light some clues behind the city's destruction.