The Attenbury Emeralds by Jill Paton Walsh
It was 1921 when Lord Peter Wimsey first encountered the Attenbury emeralds. The recovery of the magnificent gem in Lord Attenbury's most dazzling heirloom made headlines - and launched a shell-shocked young aristocrat on his career as a detective. Now it is 1951: a happily married Lord Peter has just shared the secrets of that mystery with his wife, the detective novelist Harriet Vane. Then the new young Lord Attenbury - grandson of Lord Peter's first client - seeks his help again, this time to prove who owns the gigantic emerald that Wimsey last saw in 1921. It will be the most intricate and challenging mystery he has ever faced ...Since the publication of A Presumption of Death, which was set in 1941 in the wartime English countryside, readers have been eagerly asking for this story - a wholly original and utterly engrossing new detective adventure.