We got a lot of original mecha stories around the turn of the millennium, most of which were anime originals. This is one of the rare exceptions (even if it got an anime adaptation later down the line).
BOKURANO OURS (Bokurano), by Mohiro Kitoh. First published in 2003 and first published in North America in 2010.
PLOT:
It's an ordinary summer in an otherwise unremarkable seaside town. A group of kids are wandering around the beach when they discover a cave in the cliffside. Inside that cave they find a strange man in a makeshift room loaded with machinery. The man asks the kids if they want to play a game, but asks them to pledge to keep it secret. The kids don't take the situation seriously at the time, but everything changes when they are whisked away from their classrooms and into a sparse, white room inside a giant insectoid robot. It's only then that the man explains the rules of the game: each child will be given a chance to pilot the robot to fight other alien robots, with the fate of the world on the line. The kids win their first battle, but in doing so expose some of the emotional fault lines within their friend group with deadly results.
