A Spoonful of Poison by M. C. Beaton

Agatha Raisin, private investigator, is persuaded by the vicar to help publicize the church fete and she agrees. Several offerings in the jam-tasting booth turn out to be laced with LSD and a couple of little old ladies die because one couldn't fly and another drowned. What a lot of rubbish. Wouldn't waste my time on another of her books. Sandy Penton