In Passing Strange (Reviews, Dec. 1), Sandweiss uncovers the double life of Clarence King, the renowned geologist who mapped the American West—and crossed color lines, passing as a black Pullman porter, James Todd, to marry Ada Copeland, a black nursemaid.

How did you come across this story?

I read Thurman Wilkins's spectacular biography of Clarence King in graduate school. It haunted me. Reading Philip Roth's wonderful The Human Stain got me thinking again, as did the Clinton scandal: Clinton messed up for 30 minutes and the story was spread around the world. King sustained a secret life for 13 years. So I sat down one day to see if there was anything I could do with the story. In five minutes I found the census document of King reporting to be black.

King's life reads like a Who's Who of the time: Henry Adams, Henry James, John Hayes—

James Weldon Johnson! Who would have thought? And Frederick Douglass. It's like six degrees of Clarence King. In the 19th century, he would have been on the cover of [the equivalent of] People magazine, but when the story came out in the 1930s, he had dropped out of public consciousness.

It's difficult for modern readers to understand how King could pass as black. What have we forgotten about how racial distinctions used to be in America?

We're still dealing with the legacy of the one drop of black blood rule. Think about our new president. We call him black because 50% of his ancestry comes from an African man. We don't imagine him existing in an intermediary category. The fact that we're so quick to call him black speaks to the lasting legacy of the rules that enabled Clarence King's passing. It doesn't depend on what you look like. It's so ironic that these racial laws sprang up during Reconstruction in order to fix racial identity—to say that if you have one black great-grandparent, you're riding in the Jim Crow cars—but they made race more fluid. They let a man like Clarence King, who had no African ancestry, claim that he did.

Racial classification must have seemed so arbitrary to Ada King. Her daughters later passed as white, yes?

It's remarkable how white their daughters were, or the daughter that I have a picture of. In the photographs [in the book], you can see how she could marry as a white woman. Normally, I like to have pictures integrated into the text, but in this case, since there's no photograph of Clarence and Ada King together, it would have been hundreds of pages in the book with only Clarence and then pictures of Ada and her family during the trial. That didn't feel right. I begged the publisher for a photo insert, because I wanted readers to see them together, to look at him and look at her and wonder: how did they pull this off?