WE CANNOT STOP talking about Sylvia Plath. Or rather, talking about Sylvia Plath has become a way of talking about other issues - issues which are unrelated to her poems themselves, issues which are precisely to do with what it is we are allowed to talk about. Those who write on Plath, whose own writing, as her husband Ted Hughes put it, went straight for the 'central, unacceptable things', find themselves confronting questions of acceptability too: about the acceptable limits of biography (how much it is possible to say about the marital breakdown of Plath and Hughes); the acceptable, ethical limits of editorial control (Hughes destroyed one of Plath's journals, severely edited her work, and authorised the publication - or not - of all her posthumous writings); the acceptable limits of interpretation (what it is possible to say about Plath's poems without causing offence to the estate). What it has been possible to say about Plath - about her six years with Hughes, her suicide in February 1963, those lethal last poems, which Hughes discovered, put together and ordered to form the work which sixth-formers and students all know as Ariel - has been controlled by the fiercely protective Olwyn