Another sure best seller, with all the earmarks of a book that should have permanent adult value as well as juvenile. Out of a wide choice -- almost immeasurable -- Van Loon has selected 24 folk songs, representing a far reach of interests and backgrounds and an historical sweep that drives home the fact that folk songs are inherent in racial and social history. In his foreword, he gives one a sense of time, place, setting, people and the stories back of them. The pictures are typically Van Loom. Miss Castagnetta has ably done her share in the musical arrangements.