Classified as Murder (Cat in the Stacks Mystery) by Miranda James

Librarian Charlie Harris and his cat Diesel must catalog a killer in this mystery in the New York Times bestselling Cat in the Stacks series.

Suspecting that someone is stealing from him, the aging and eccentric James Delacorte wants Charlie to do an inventory on his rare book collection. Soon after they begin, Delacorte is found dead at his desk, leaving Charlie with the bigger task of solving his murder.
 
Immediately Charlie is suspicious of Delacorte’s own family, and relies on the help of Diesel to paw around for clues. The cat and mouse game heats up after a highly valued copy of Edgar Allan Poe’s Tamerlane goes missing and a second murder occurs. Now Charlie and Diesel must solve the case before the killer strikes a third time—and hope curiosity doesn’t kill the cat...