Briar's Book is the last installment of the Circle of Magic quartet by Tamora Pierce. The main protagonist of the book is Briar Moss, a young ex-thief and "green" or plant mage, having ambient magic with all forms of plant life. Through his eyes the book explores themes of poverty and social injustice, as a deadly plague named the blue pox strikes The Mire, the poorest quarter of Emelan's capital city, Summersea. The Blue Pox was created by a low level female mage trying to make a weight loss potion. Her efforts were not successful and she disposed of the magical wastes incorrectly and the magic seeped into the water supply. From there, the magic combined with other substances to create the deadly Blue Pox.