In 1964, the El Mochuelo Gallery of Santa Monica published a major monograph on Callahan's work. Consisting of 126 images, it was issued in a limited edition of 1500 copies. The "El Mochuelo" prints are rare as most are now in museum collections. Almost concurrently, Hallmark Cards purchased a large group of prints from Callahan, after exhibiting them in their New York showcase gallery.