For 15 years, America's most popular living artist worked in secret with neighbor Helga Testorf as model. The resultsome 240 pencil sketches, watercolors, drybrush, and temperas, concerned with Helga in all aspects, nude and clothedare here shown in 100 high-quality color plates and 160 black and white illustrations. Works in progress reveal the artist's methods; finished works, an obsession with his model as awesome as his technique. Wilmerding, deputy director of the National Gallery, contributes an informative text, further clarified by the artist's own observations. Gloria K. Rensch, formerly with Vigo Cty. P.L., Terre Haute, Ind.