'The Body In The Library' by Gordon Lawrie

Holmes studied the victim’s body in the library. Head beaten with a blunt instrument. Blood, yes, but also water on the carpet. No murder weapon.
“I’m baffled,” Watson said. “It’s cold in here.”
Suddenly, Holmes leapt to his feet. “It’s elementary, Watson!”
“Elementary?”
“The butler did it,” said Holmes. “Ring for him!”
Moments later the butler, Jeeves, appeared. “You rang, sir?”
“You beat the victim to death with a block of ice, which melted.  As butler, Jeeves, you’re the only one with access to ice.”
Jeeves bowed. “As you wish, sir.” Then he added, “Shall I bring the handcuffs, sir?”