Who is Great-Great Uncle Arthur from Lucinda's Secret and what is their importance?

Great-Great Uncle Arthur wrote a Field Guide about the Fantastical World, its creatures, and spells that work in the real world and the Fantastical World. The Field Guide has caused others to be in danger in the past, and it is causing the Grace family to be in danger now. Arthur leaves one day, and he is never heard from again. It is assumed he abandoned his family. Over the course of the novel, the children learn that he is being kept alive by the Elves in the Fantastical World.