In 1992, Rasmussen took the position of assistant to S. Dillon Ripley, the former secretary of the Smithsonian, who was planning to produce a definitive guide to the birds of South Asia. When he became ill shortly after beginning the project, Rasmussen took over the project, and with artist John C. Anderton, produced ', a two-volume bird guide for the Indian subcontinent which was the first field guide for the area to include sonograms. Volume 1 contains the field guide with over 3400 illustrations in 180 plates, and more than 1450 colour maps. Volume 2 ("Attributes and Status") gives specimen measurements, data about identification, status, distribution and habits. Vocalizations are described from recordings, and there are over 1000 sonagrams.