Jack Janson and the Storm Caller

Jack Janson is nearly fourteen, an only child living with his parents who hate him almost as much as they hate each other. The only good things about his life are the girl next door, Sarah-Jane Farmer, whom he adores, and his Granny Jean in Cornwall who he spends the summer holidays with.
His gran is cool but she has been hiding a HUGE secret. As her health fails, she decides to share the secret with Jack.
Gran leads Jack to a cave.
"Boom Tom tum" a loud voice echoes and a rock opens up to reveal a young giant called Winfred Storm Caller. Gran has been looking after the friendly giant since pirates killed his mother, but she now needs Jack to care for Winfred.
Sarah-Jane arrives to help and they uncover The Book Of Lore hidden in the cave.
What magic does it possess?
Have they found a way to get Winfred home to his own lands?
Are Sarah-Jane and Jack brave enough to use the book to save Granny Jean's life?