The New York Times bestselling author of The Monogram Murders and Woman with a Secret returns with a disturbing tale of psychological suspense and obsession that hits at the heart of some of our most precious relationships.

What if having a best friend could put you in the crosshairs of a killer?

A psychopath the police have dubbed "Billy Dead Mates" is targeting pairs of best friends, and killing them one by one. Before they die, each victim is given a small white book.

For months, detectives have failed to catch Billy, or figure out what the white books symbolize and why the killer leaves them behind. The police are on edge; the public in a panic. Then a woman, scared by what she's seen on the news, comes forward. What she reveals shocks the investigators and adds another troubling layer to an already complex case.

Stand-up comedian Kim Tribbeck has one of Billy's peculiar little books. A stranger gave it to her at a gig she did last year. Was the stranger Billy, and is he targeting her—or is it something more nefarious? Kim has no friends and trusts no one, so how—and why—could Billy Dead Mates want to target her? If it's not her, then who will be the next to die?

Sophie Hannah raises the stakes with each successive page in this haunting and twisting thriller that reaffirms her place as one of today's most talented suspense writers.