Chrysoberl is an artist just barely making her rent when she receives word: the medical experiment she volunteered for is ready for her. To her surprise, instead of a new drug she gets an entire race of microbial people who live inside her brain, patrol her body for ill-health, and worship her as a god. She and the microbial people enter into a tentative detente--she will feed them arsenic and give them light, and they won't turn her into a slave using their ability to manipulate her sensations of pain and pleasure.
This is the fourth and possibly last book in Slonczewski's acclaimed Elysium cycle, a series that spans a number of worlds and hundreds of years, yet never lost its personal touch. Like all of the books, the main character has personal problems and concerns, yet is still involved in a much larger social change or revolution taking place. And like the others, this book features a unique mix of hard sf (Slonczewski is fantastic at using biochem to create realistic aliens and future tech without ever infodumping) with a thoughtful exploration of morality.