Nod's Limbs by Charles Ogden
It's all-out war with the Knightleigh family, and Edgar and Ellen are losing ground. But everyone's attention is diverted by the ghostly hand of fortune: Augustus Nod has launched a treasure hunt from beyond the grave! The twins must solve the riddles and discover Nod's lost golden limbs before the Knightleighs bury the past - and the twins with it!