“Once per decade we are graced with a new book by Otsuka, the award-winning author of 2012’s “The Buddha in the Attic” and 2003’s “When The Emperor Was Divine.” This year’s novel starts as a catalogue of spoken and unspoken rules for swimmers at an aquatic center but unfolds into a powerful story of a mother’s dementia and her daughter’s love. If Otsuka doesn’t write another novel for several years, it will be okay. This is one to be savored and reread.”