''Documentary fiction'' is what Iain Sinclair oxymoronically calls this book. It's a lot of other things too: autobiography, history, psychogeography to name but three. His ''Hackney book'' as he self-referentially calls it throughout, is a dense collage of reportage and ''inaccurate and inventive'' transcriptions of interviews, peopled by film-makers, novelists, politicians and painters, not to mention booksellers, barbers and bus drivers.