Book Briefing -- recent nonfictions; The Florence Baptistery Doors, photographs by David Finn, text by George Robinson, introduction by Kenneth Clark. New York: The Viking Press. $50.

The Florence Baptistery Doors" is as much a tribute to the fine art of photography as to one of the foremost surviving examples of medieval bronze relief sculpture.

The audacious objective of photographer David Finn was "to rediscover in all three doors what the sculptors themselves had seen. . .. I wanted to follow their eye . . . seeing the figures not only straight on from the front, but from above, from behind, from the side."

That he did, as one finds from the book's 321 illustrations, 42 of which are in color. Teetering on a ladder and crouching on his kees, Finn captured wonderful details that are virtually invisible to the ordinary onlooker.

As for the magnificent doors themselves, containing scenes from the life of John the Baptist and from the Old and New Testaments, art historian Kenneth Clark notes that they (together with a few other bronze portals) are nearly "the only complete works of art that have survived from the Middle AGes undamaged and unrestored. . . ."