'To admit to writing about artists' models is to set off an avalanche of interest. ''Didn't Rossetti marry his model?'' ''didn't Augustus John sleep with all of his?'' At first I brushed such questions aside as frivolous. Instead of revealing the facts about modelling in England from the foundation of the Royal Academy to the present day, the gossip column approach to art history seemed to veil them. But as research revealed the mundane business of...