BERN, Switzerland—Albert Hofmann realized he had invented LSD after a vivid experiment in 1943. The Swiss chemist retired a few decades later, and his personal archive began a long, strange trip that ended at a quiet institute in this leafy city—where it is looked after by a part-time dairy farmer.

Beat Bäche, who is writing a book about hallucinogen-producing fungus when he isn’t milking cows at a farm where he works, curates Dr. Hofmann’s papers. That is because Mr. Bäche is nearly the only person to use the archive since...