Berries

Vermont--

His breath smells of juniper berries which give gin its distinctive flavor.

Sue smells Behrman, and Behrman has the smell or odor of juniper berries. That's why he "smells strongly of" juniper berries. If he was smelling the berries, the words "strongly" and "of" would be left out, and the sentence would read,

Sue found Behrman smelling juniper berries in his dimly lighted den below.

May I add that juniper berries are not generally eaten; they are used to flavor gin. I think the author is trying to tell us that Behrman has been drinking alcohol.