Gussy, by Jimmy Cajoleas

Gussy has been training to be a village protector for as long as she has lived with Grandpa Widow.  It's an important but tedious job as every ritual and rite must be performed with absolute precision in order to prevent the Great Doom from reaching them.

She's still learning the practices, but she's confident she can manage to keep things together for a few weeks when Grandpa is called away.  But on the first night, Gussy allows a refugee to enter the village, opening up the gates that must never be opened at night in order to do so.  She knows it's a mistake, but the refugee is a harmless little girl and poses no threat.

After that, things start to go bad.  Objects become possessed with evil and then a force starts taking over humans as well.  The Great Doom has breached their walls and defied all of the wards and spells that protect the community.  Gussy exhausts her knowledge of magic and protection rites, but the darkness are still descending upon them. If only Grandpa would come back, but there's no sign of him and Gussy knows that she'll have to figure out a way to defeat this evil that she may of unwittingly brought upon the village.

Excellent world-building and a strong and clever heroine with a lot of mojo gives us a decent (albeit fairly predictable) fantasy novel.  The storytelling drags at points and overall it may be a bit too cerebral for its targeted middle school audience, but the tale checks off all of the right boxes.  There are some good messages about the power of good teamwork and the importance of not holding on to grudges tossed in as well.