Currently ten translated chapters are out for the manga. And two light novels currently translated on bakatsuki
Just so you know, Each High Level Piece has a value of multiple pawns. (Other then King of course)
King = Owner of other pieces and distributes them. Strongest position (From what I read so far)
Queen = Equal to 9 Pieces
Ability: Rook + Bishop + Knight
(2)Rook = 5 Pieces
Ability: High ATK and DEF
(2)Knight = 3 Pieces
Ability: High Speed
(2)Bishop = 3 pieces
Ability: (Doesn't really say. Though I'm guessing its higher magic stats)
(8)Pawns = 1 Pieces each.
Ability: Can make a temporary class change to any piece other then king when in enemy territory. (I think King Decides where that is exactly)
You can tell how strong the pieces are based on how much they cost. Certain people require more pieces in order to be resurrected. Like:
Main Character Required 8 pawns
From what I read from the manga and light novel the one being turned into a piece has to be strong enough to handle the power increase.
Main Character had a seal on him because apparently having the power of all 8 pieces would have overwhelmed him. Its suggested that no all of them are unsealed.