Under the Green Hill - Laura L. Sullivan
Meg and her siblings have been sent to the English countryside for the summer to stay with elderly relatives. The children are looking forward to exploring the ancient mansion and perhaps discovering a musty old attic or two filled with treasure, but never in their wildest dreams did they expect to find themselves in the middle of a fairy war.
When Rowan pledges to fight for the beautiful fairy queen, Meg is desperate to save her brother. But the Midsummer War is far more than a battle between mythic creatures: Everything that lives depends on it. How can Meg choose between family and the fate of the very land itself?