Henry James, the author of The Portrait of a Lady and other literary masterpieces, becomes an unlikely action hero in this ambitious Sherlock Holmes pastiche from Simmons, a writer I’ve been meaning to read for a long time. I was duly impressed. Other Doyle disciples have teamed Holmes with eminent figures of the Victorian age (perhaps most notably Nicholas Meyer in The Seven Per-Cent Solution, in which Holmes gets psychoanalyzed by a young Sigmund Freud) and have purported to reveal the great detective’s dark side as the proper Doyle would never have done (see Michael Dibdin’s The Last Sherlock Holmes Story, about Holmes and Jack the Ripper), but Simmons takes this game to a new level.

In a meta-fictional twist, Holmes is on the verge of suicide because he fears he’s a fictional character. A chance meeting on a Paris bridge in 1893 results in the likewise suicidal James joining Holmes on a mission in America, where the Baker Street sleuth hopes to prove that historian Henry Adams’s emotionally fragile wife, Clover, was not a suicide but a murder victim in 1885—and to thwart an international conspiracy involving an attempt to assassinate President Grover Cleveland at the opening of Chicago’s Columbian Exhibition.

The leisurely, digressive plot contains a number of clever twists and surprises, and how James evolves in his Watson role is one of the book’s highlights. But for me, the real pleasure lies in such incidentals as James’s extended commentary on the shortcomings of “The Adventure of the Copper Beeches,” one of the most famous Holmes stories. And in one amusing exchange, James and Mark Twain discuss whether they’re characters in a novel and, if so, who might be the author. Twain says to James: “It’s almost certainly some lesser mind, lesser talent, than you, than me, even lesser than Arthur Conan Doyle, which is saying a lot. And it might be written thirty years hence, or fifty, or a hundred.” Who can resist such a self-referential putdown?