Florentines on the eve of November 4th were preparing to celebrate Armed Forces Day but events had other occupations in store for them. The first word on the flood came from the Civil Engineering Department around 2:30 a.m.--""an exceptional quantity of water..."" Mr. Nencini records what that exceptional quantity of water did in pictures that show the flood at its height, swirling about the Baptistery, submerging the cloisters of the Santa Croce, the Ponte Vocchio, and on throughout the stricken city. His text narrates the onrush, the reactions...in prisons, hospitals, shops and homes. There were the immediate problems of escaped prisoners, babies in incubators with the generators out of commission; later, bread and water and rooftop rescues. Mr. Nencini discusses the shortcomings of the state in dealing with flood control, records other historic floods. He depends on the drama of the circumstances for his effects; his writing is pedestrian, dealing with an exceptional occurrence unexceptionally, if competently, and one wonders whether an American audience, even those who love Florence, will respond.