Appleby and Honeybath

From Goodreads: "Every English mansion has a locked room, and Grinton Hall is no exception - the library has hidden doors and passages and a corpse. But when the corpse goes missing, Sir John Appleby and Charles Honeybath have an even more perplexing case on their hands - just how did it disappear when the doors and windows were securely locked? A bevy of helpful houseguests offer endless assistance, but the two detectives suspect that they are concealing vital information. Could the treasures on the library shelves be so valuable that someone would murder for them?"