Crackerjack children's writer Jean Fritz here presents an intriguing dual biography: first, of Leonardo da Vinci, who labored for years on a giant bronze sculpture of a horse, never finished; and secondly, of Charlie Dent, an American who in 1977 vowed to complete what Leonardo began. Dent, too, died without seeing the horse finished, but his wishes were honored and the giant horse became a gift to Italy from the United States. Today it's on display in Milan. The book is illustrated, handsome and appropriate for a wide range of readers, from 7-year-olds to adults.  (ITL344, $18.99)