‘To my mind,’ Renoir once wrote, ‘a picture should be something pleasant, cheerful and pretty. There are too many unpleasant things in life as it is.’ What would he have made of Edward Brooke-Hitching’s The Madman’s Gallery? Of the 100-plus artworks it examines, few are cheerful and fewer pretty. Often you turn the pages of an art book wondering which painting you most covet, but with this one it’s more a question of which you’d pay not to own: the 13th-century ‘Penis tree’ maybe? The 90 cans of excrement sealed up by Piero Manzoni in 1961 and now selling for...