There are dozens upon dozens of advice books for teenaged girls. Girls love to share. Boys are famously somewhat less forthcoming - unless it's with a bit of healthy bullsh... er, exaggeration. Girls are also more likely to ask for help than boys, who feel their reputation rests on knowing it all already. So ''Unzipped'' is a very useful little volume. It covers all the main subjects - puberty, the body, personal hygiene, parents, relationships, sex, drink and drugs. And it does it in good, plain English and a supportive, encouraging tone.