The Green Children of Woolpit, by J. Anderson Coats

Back in the 12th century, two green children were found in the English village of Woolpit.  The boy died, but the girl lived, and spoke of the twilight underground country they'd come from.  They became a legend.  And now J. Anderson Coats has made them the center of a magical middle grade story, The Green Children of Woolpit (Atheneum, September 2019).

Except that the center of her story is not actually the two green children, but Agnes, the peasant girl who finds them.  Agnes, whose mind wanders, who can see the wind, who isn't rushing toward growing up like her former best friend, Glory.  Agnes was the only one to hear the green children calling for help.  And because she went to their aid, her own life becomes a nightmare.

While the green girl tries to take her place in her family with guile and fairy glamor, Agnes is trapped in the underground halls of the malevolent and sadistic Good People.  To make things right, she must undo the bargain she became ensnared by, but it is a very tricky business to try to outsmart the fairies....

It's top notch historical fantasy, with lots of shuddery horror and magic.  It's not a swords and sorcery sort of fantasy, but a more personal journey, though one full of magical dangers.  Agnes, and the green girl too, both become very real for the reader, and I found their struggle to take back their lives from the Good People totally engrossing.