Unique German editions of the Three Investigators (Die Drei???)  books: #23 Mystery of the Invisible Dog (Karpatenhund), #25 Dancing Devil (Tanzende Teufel), and #37 Two-Toed Pigeon (Perlenvögel).

Books are fully in German and in very nice shape. Tanzende Teufel is missing its dust jacket, but the other two both come with dust jackets.