His Psyche (1890-1894) remains a standard reference work for Greek cult practices and beliefs related to the soul.
His work, Der Griechische Roman und seine Vorläufer (1876), was considered by Mikhail Bakhtin to be "the best book on the history of the ancient novel", and it is still regarded as one of the greatest "monuments of 19th century classics scholarship in Germany".
Psyche: The Cult of Souls and the Belief in Immortality among the Greeks, trans. from the 8th edn. by W. B. Hillis (London: Routledge & Kegan Paul, 1925; reprinted by Routledge, 2000), limited preview online; full text in German downloadable as pdf.