Meat for Bemelmans' fans, in the groove with How to Travel Incognito, quite mad and utterly beguiling. Its extra dimension this time is the omnipresence of irrepressible Barbara, who at thirteen is an extension of all the one track mind females who know what they want and go after it. She and her father take a European trip and encounter all sorts and conditions of things and people. Sudden bits of serious philosophy relieve; it of the charge of sheer insanity.