There's the nostalgia of an adult remembering childhood, with the intensity of its agonies its emotions, its enchantment, its gradual awareness, in this new book by the author of Forever Amber. The span of a year spent on Laurel Avenue, on the edge of a Western town, gives Cassie Spangler, who leaves 12 behind her, regretfully, a full measure of life. She and her small brother, Don, make their places with the gang quickly; Cassie and her pals are still experiencing the imaginary thrills of what they think being grown up will be, and sex is for them chance words overheard, an embarrassment over their parents' affectionate interchanges, indignation when a young married couple quarrel openly, fleeting curiosity over what happened when two teen agers went too far and the girl's family moved away, excitement when the sailor boy came home and claimed his girl, only to find she'd not stayed in the house alone in his absence, anger when death took a young mother-and her husband found another wife too soon. Cassie learns too something of what responsibility means- and faces another move, chin up. Almost a keyhole segment of America.