I left Egypt in 1921 at the age of twelve and have never been back,"" Priscilla Napier begins her recall of the years before. But it is not Egypt on stage here, but the state of childhood, from the time when at two she took the Great Pyramid to be the primal wonder (beside herself) to the time when at twelve she climbed it (and realized that other people had existences of their own). Mrs. Napier deals with that period of delighted self-centeredness with a lovely pragmatic humor that makes tactile all the habiliments and inhabitants of a child's world-- the ministrations of Nannies, the amused laughter of elders, the collusion of cousins, days playing at being an Antelope or a Norse God, running away or drinking forbidden Nile water. Her father, a legal and sometime financial adviser of the Egyptian Government, saved her mother from a Victorian daughterhood in her late thirties, and welcomed her to the country he served well enough to earn a knighthood (for which his father roundly put him down-- only an earldom would do). The only shadow over her childhood was the fact that other cousins lived in India, and if there were dead people in the Ganges. This kind of one-up-child-manship is curiously carried over in other books, notably Jon and Rummer Godden's mythic Two Under the Indian Sun, basking on the best seller list. Mrs. Napier will have her own, perhaps lesser following, but her ""under the Egyptian sun"" need take second place to none.