Ann Landers may not be everybody's cup of tea, but she sounds like a neighbor who would drop in and share one to the thousands who write her awkwardly worded pleas for public advice on private problems. In discussing the teenage troubles of low-budget eduction (try now, pay later), homosexuality, liquor and venereal disease, she uses the etters that come to her widely syndicated column in order to reinforce her points. She arrives at the side of the angels without sermonizing -- hers is the vulgate of virtue. She says what clerics, psychologists and social workers have been saying for years to non-listening, non-readers who can take and apparently like it from her. The book is addressed mostly to teenagers with occasional asides to parents. This split approach is unfortunate, but not unrealistic in terms of her potential audience -- miserable mothers with distressing daughters. She likes teenagers and it shows. She concedes the fact that messy mistakes are made but urges on everybody -- guilt givers and guilt laden --the right to a second chance in a do-it-yourself rehabilitation program. Hearty doses of hope in easily swallowed prose.