A Figure of Speech — Norma Fox Mazer

Both overlooked in the middle of a big, noisy family, Jenny and her grandpa will always have each other to confide in . . . right?

No one in Jenny Pennoyer's family understands her at all -- no one, that is, except her grandfather, who lives in an apartment in the basement of her family's home. Jenny and her grandfather have been close ever since she was born, when Grandpa, newly widowed, found that a baby was just the thing he needed to get back on his feet.
 
But as Jenny's family grows and they're all pinched together in one house, her parents become less and less patient with Grandpa's desire to be independent. Jenny feels like his only defender, the only one who sees him as a person with a mind of his own.
 
As Jenny grows increasingly protective, Grandpa's determination and Jenny's love for him will lead them on an adventure together that their family never expected.