Irving Johnson had the right idea when he married a first-class secretary to write books about the places to which they sailed in their ketch Yankee. Mrs. Johnson is as indefatigible a researcher as any lass on Time-Life. In this the third installment of the peregrinations of the Yankee, the Johnsons sail from Alexandria to Cairo and down the Nile (or up, since water flows toward the equator.) Their present book is seven-tenths historical research into the dynasties of Egypt and three-tenths local color. Fortunately, Mrs. Johnson, who does nearly all the writing, has an excellent sense of the past and a gift for 3000-year-old gossip. If you buy this book, it's for its deep enjoyments of the nightlife and political peccadillos of Cheops, Thutmose, Queen Hatshepsut, Akhnaton, Horemheb, Ramses, Cleopatra, and for passing notes on Farouk and present-day land reform programs, and Nefertiti. To be sure, the pages are sprinkled with loads of eau-de-Nile, with passing beggars and intolerable bridges that often refuse to open and permit passage. The Johnsons meet pleasant companions and observe with respect their Egyptian retainers. But mostly the trip was uneventful and, though the library at Alexandria was burned to ashes, the library at 42nd Street and Fifth Avenue offers much the same observations about Egypt as Mrs. Johnson.