Fablehaven
Grandpa Sorenson is Kendra and Seth's grandfather on their father's side. He is the caretaker of Fablehaven. Grandpa leaves clues about the true nature of the preserve, in hopes that Kendra and Seth will discover the truth that it is a preserve for magical creatures. The kids do figure it out, but soon afterward Seth, in a moment of bad judgment, makes it possible for unsavory creatures to enter the house, and Grandpa and Lena are kidnapped. They are held hostage by the witch, Muriel, at the Forgotten Chapel where she is endeavoring to free a demon named Bahumat. Grandpa is married to Grandma Sorenson who spends much of the first book in the form of a chicken.
Read more about this topic: Stanley Sorenson, In The Books