AFTER NINETY, by Imogene Cunningham (University of Washington Press, $14.95). As we grow older, most of us become obsessed with youth. With the insight of a great artist, however, Imogene Cunningham realized that it is not youth but life that matters. This compilation of 107 photographic portraits of nonagenarians is Cunningham's testimony to the potentialities of old age. Thus some of the photos were taken over the last 40 years, most were made in the year before the photographer's death in 1976, when she herself was 92 years old. The same sharp-focus candor that characterized the best of Cunningham's work when she was a member of the f.64 group in the 1930s can be seen here. Every wrinkle, every mark of aging is visible, but it is the sitter's vitality that lingers in the viewer's mind. Instead of the stereotypical portrayal of the elderly as benign, serene presences, already beyond the demands of this world, Cunningham gives us a group of individuals very much engaged with life - energetic, interesting, at times even belligerent. This strong and important book is a fitting requiem for a photographer whose talent never let her rest in peace.

FACES: A Narrative History of the Portrait in Photography, by Ben Maddow (New York Graphic Society, $35). Any survey of the portrait in photography must necessarily review the history of the medium as well, for what photographer could ever entirely resist the drama of the human face? Beginning with the elusive, silvery surface of the daguerreotype, Ben Maddow traces the changing relationship between the photographer, the sitter, and the viewer up to the present day. The most famous portraits are, of course, included - Southworth and Hawe's depiction of the sublimely decadent Lola Montez, Edward Steichen's steely view of J. P. Morgan, the great Farm Security Administration series made during the Depression, Diane Arbus's gallery of outcasts. There are also, however, pictures that rarely make the standard studies. Lewis Carroll's amatory photos of little girls should make us all read Alice in Wonderland with new insight. John Thomson's 19th-century pictures of Chinese society offer a fascinating glimpse of a vanished civilization. Maddow's commentary runs parallel to the pictures but is less a specific analysis of the work than a lyrical and highly personal reflection on man and the machine he invented to gain immortality.

EDWARD WESTON NUDES, With a Remembrance by Charis Wilson, (Aperture, $20), Kenneth Clark has written that any nude without "some vestige of erotic feeling . . . is bad art and false morals." The photographer Edward Weston is not guilty on either count. His astonishing, luminous female nudes, made from 1918 through the late '40s, are great works of art, and their eroticism is straightforward and unaffected, permeating the pictures like the silvery glow of the platinum Weston used in printing his negatives. These are very distinctive women, with different types of bodies, different responses to the camera and to the man behind it, and it is this specificity, this grounding in reality, that gives the pictures a strong sexual content. What makes these photographs art, however, is not just their eroticism - though surely that is part of it - but the manner in which Weston pushes each form toward its purest, most refined state. Without diminishing his subject's individuality, the photographer analyzes the nudes as an abstract form in space, and it is this combination of formal and sensate concerns that makes Edward Weston's nudes such an extraordinary achievement.