The Sleep Watcher, the third thoughtful novel by the gifted Rowan Hisayo Buchanan, features a narrator who floats free from her body at night and circles around invisibly, observing her family and friends. This departure into the supernatural from the author’s previous work does not leaven the sadness of her writing, and the book is even more melancholic than her Starling Days (2019), which opened with the protagonist contemplating suicide.