The RHS is the International Cultivar Registration Authority for the genus Clematis
Download registration forms here:
Application form to register a Clematis cultivar name (pdf)
Application form to register a Clematis cultivar name (doc)
The RHS publishes The International Clematis Register and Checklist 2002 and subsequent supplements, some of which are available as downloads from the link below.
Evison, R. 1985. May Large-flowered Clematis cultivars. The Garden 110(5): 207-213.
Fretwell, B. 1989. Clematis. Deer Park, Wisconsin: Capability's Books.
Lloyd, C. 1989. Clematis. Deer Park, Wisconsin: Capability's Books.
Rogerson, B. 1985. Clematis viticella and its progeny. Pacific Horticulture 46(1): 40-44.