“These stories give readers permission to think about what it would be like to wake up as another gender or another race,” Mr. Cooper said. The work is a plea for empathy, he added, “an investigation of what it might mean literally to step into the shoes of another.”

One may learn, as Drew does, that being pretty and popular is no shield against the envy or predations of her nastier peers. One may encounter, with Oryon, the reserves of fear and bias that are only partly concealed by a veil of civility.

Oryon is compelled to sit in the school lunchroom exclusively with other black students and endure racial profiling at the hands of the local police; he is roughed up by members of a faction known as the Abiders, self-appointed keepers of the status quo. “If you learn anything as a Changer,” he observes in “Book Two,” “it’s that all the supposedly bygone stereotypes and prejudices are far from bygone.”

For Mr. Cooper, the book provides an opportunity as well to sort out and clear up misconceptions about what it is to be transgender. “That doesn’t entail just a change of name or surgery or clothes or haircut,” he said. “For so many people it is hard to understand that a man can be attracted to women but then also feel like a woman. Once he transitions into a female, that doesn’t mean he suddenly wants to be with men.”

That observation is reflected throughout the series. “Somebody who has to live through four different iterations,” Mr. Cooper said, “learns that gender doesn’t really affect what or whom he’s drawn to.”

Each book is a probe into effects of gender, race and body image on the formation of identity. Those externals, along with the experiences accrued in a lifetime, do little to alter that immutable bedrock known as character, the authors argue. “Each of the books is a meditation on the essentialness of humans,” Mr. Cooper said.