Murder Between the Pages — Josh Lanyon

Felix Day, author of the Constantine Sphinx mysteries, and Leonard Fuller, author of the Inspector Fez mysteries, are bitter rivals and the best of enemies. Both happen to present when a notorious author of roman à clef is shot by an invisible assailant during a signing at historic Marlborough Bookstore.

Even if they weren't both suspects, it's the perfect opportunity to match wits and sleuthing skills.

If only the murderer was equally amused.