Free again from the author via KDP Select @
Amazon:
Death of a Lovable Geek by Maria Hudgins, 2nd in her Dotsy Lamb Travel Mysteries starring a history teacher as the amateur sleuth, originally out from Five Star in 2008.
This is actually not filed under the Amazon cozy categories, but several customer reviews for this and others in the series refer to them as cozies, with one reviewer specifically likening this one to the "good harmless fun" of Agatha Christie stories. YMMV.
(And IIRC, forum moderator dreams read and liked either this or one of the other ones in this same series in the What are we reading? thread over in Reading Recommendations a couple of years back.)
The body of young Froggy Quale lies, covered by a blue tarp, in the pasture behind Castle Dunlaggan in the Scottish Highlands. Froggy was the on-site pollen and spore expert at the nearby archaeological dig and a graduate student who doesn't seem to have had an enemy in the world.
Dotsy Lamb, ancient and medieval history teacher from Virginia, and her friend Lettie are boarding at the castle while Dotsy works at the dig. They and the other guests are cared for in the ancient castle by its owners, William and Maisie Sinclair.
Suspicion for Froggy's murder falls on his roommate, Van Nguyen, a graduate student from America. Dotsy has become fond of both Froggy and Van and is sure Van can't be the killer. When the dig's director dies from poison mushrooms, Dotsy sees connections to the magic mushroom party at the dig and to ownership of castle lands.
This is a suspenseful brainteaser that pits the brains of an American history teacher against the dark secrets of a tragic family history.