Possible Clues:

• Master under servant in great book
• 'Unputdownable' book
• Wild ape, grunter in exciting tale
• Book for Ellen and Ted?
• Can't put it down to pianist's assistant?
• Book one can't put down
• Mature artist put under pressure to produce an exciting book
• Exciting work uniting assistant and painter
• Enthralling work required, send for artist
• Summon artist, producing an absorbing book