Summary from the publisher:
An epic fantasy about a young girl raised by a witch, a swamp monster, and a Perfectly Tiny Dragon who must unlock the powerful magic buried deep inside her.

Every year, the people of the Protectorate leave a baby as an offering to the witch who lives in the forest. They hope this sacrifice will keep her from terrorizing their town. But the witch in the forest, Xan, is kind and gentle. She shares her home with a wise Swamp Monster named Glerk and a Perfectly Tiny Dragon, Fyrian. Xan rescues the abandoned children and deliver them to welcoming families on the other side of the forest, nourishing the babies with starlight on the journey.

One year, Xan accidentally feeds a baby moonlight instead of starlight, filling the ordinary child with extraordinary magic. Xan decides she must raise this enmagicked girl, whom she calls Luna, as her own. To keep young Luna safe from her own unwieldy power, Xan locks her magic deep inside her. When Luna approaches her thirteenth birthday, her magic begins to emerge on schedule–but Xan is far away. Meanwhile, a young man from the Protectorate is determined to free his people by killing the witch. Soon, it is up to Luna to protect those who have protected her–even if it means the end of the loving, safe world she’s always known.

Review:
Barnhill begins The Girl Who Drank the Moon with characters separate from the main story, where a child asks her mother to tell her stories. The mother and child reappear intermittently throughout the book, adding to the reader's understanding of how the legends and views of the Protectorate have come to be. This familial pair helps set the stage for the following action or guides the reader into understanding the "how"s and "why"s of such actions.

Barnhill's world building is phenomenal. Sorrow is a presence in the Protectorate, its absence is pronounced in the Free Cities, and it has a tendency to be a gnat humming in the ear of those in Xan's glade. Barnhill writes a world where possibility, good and bad, is tangible at every turn.

There is much good humor and joy found in the book. Glerk and Fyrian's interactions with the child Luna often caused me to laugh out loud. It is a true coming of age story, and not just for Luna. There are many moments in the story where characters, as Dumbledore says, must choose between what is right and what is easy. It is a story of how we both are, and are not, a product of our environment. It is a story of the bonds that draw family, friends, and communities together. And it is a story of great love and great courage, especially when the Tiger is on the prowl and all that they know and love is at stake.

A fun and fast-paced read, Barnhill's book is for middle grade readers who love magic, mythical creatures, and strong female characters.