BAD SEX WRITING, LIKE A POORLY made blue film, is a repository of cliché: a world in which male members are invariably firm, breasts pert and limbs permanently entangled. But the good stuff — ah! — that has a power beyond the finest moving image, beyond, even, the memory of sex itself; it is unbounded by time constraints, regrettable haircuts and poor lighting. Written sex also has the distinction of being the only erotic art one can indulge in on public transport without fear of arrest. This is a truth well known by schoolgirls, who from the age of 13...