With a career of more than 60 years so far, Frank Auerbach is undoubtedly one of the big beasts of the British art world. His personal reticence, however, and the condensed, impacted idiom of his painting have contributed to his enigmatic, somewhat opaque reputation. Catherine Lampert, who has sat regularly and patiently for him since 1978, is uniquely qualified to throw light both on the man and his art, but the tactics she employs here are very different from those of Martin Gayford in Man with a Blue Scarf, his intimate, engrossing account of sitting for Lucian Freud.