Unsafe Hands — Jane Aiken Hodge

Until Gerald's inheritance disappeared, it had been an almost perfect day. Hoblong Hall was at its most benevolent. Violet Cunningham marshalling parishioners and family members with majestic efficiency, determined nothing untoward should upset Lesser Muchton's annual flower show.
But something did. A mysterious document -- contents unknown -- suddenly vanishes from its all-too-obvious hiding place. Detective Crankshaw, desperately inexperienced and frequently embarrassed, faces layer upon layer of Hoblong skulduggery in his quest for the truth. Then, just as things begin to make sense, Aunt Fan decides to go for a midnight walk ... armed with a shotgun.