Perri Klass. Houghton Mifflin. 276 pages. $24.

Perri Klass is a practicing pediatrician who understands the murky risks of judging a mother's care. The Mercy Rule, her sixth work of fiction, is an insightful novel comprised of stories about a doctor who runs a clinic in Boston for children who are in state custody -- or about to be. Passionate and self-deprecating, Dr. Lucy Weiss wants to slap some sense into the careless mothers who pass through her office, but she also wants to hug them and give them extra diapers.

One mother in particular, an infuriating woman who appears in several stories, pulls at her sympathies: ``There is something about Delia -- sometimes she makes me think of my child self, lost in the world and waiting to be saved, and sometimes it is her children who remind me of that self, and all in all, it makes me crazy that I am going to be the one to do what I am going to have to do: report her for neglect, for incompetence, for being Delia.''

Equally poignant and often witty are Lucy's reflections on her role as mother to a confident daughter and an odd little son she's determined to protect from the pettiness of the world. When she's not swearing to withdraw from the competition among wealthy parents, she fantasizes about beating them at their own game.

As a doctor, Klass must know the body inside and out. These stories show that, as a writer, she knows the heart and soul just as well.

Ron Charles reviewed this book for The Washington Post.