Tell your grandmother, or you could do it yourself, to look up all the following on amazon.com and read the reviews there before buying. She will see that the following books are excellent for this age group.
Get them the books of Laure Halse Anderson. "Speak" was her first and it won awards. She now has 3 novels out and they are brilliant writing. My 12 year old devours them. "Catalyst" is the second title I think.
Try also Margaret Peterson Haddix. She has a whole series about the Shadow children who survive on the outskirts of society. "Among the Brave" is one of her titles.
"Chasing Vermeer", can't remember author name, is an excellent and engrossing mystery of art theft with children as detectives. My daughter ate it up.
She has also loved the books of David Almond, a British writer whose books are available here in the U.S. They always feature sensitive children. Thoughtful pieces.
Try Louis Sacchar, especially "Holes".
Try "Star Girl" by Gerry Spinelli. Also his "Maniac Magee", "Loser" and other novels.
My daughter Martha's latest find is the series of historical novels featuring strong female characters by Ann Rinaldi. They were reccommended to her by the kids' fiction assistant in our local Borders bookstore. She can't get enough of these.
Martha also loves "The Westing Game", another solve a mystery novel. She has read this several times already.
Other great authors: Jacqueline Wilson and Eva Ibbotson. And try her with the "Molly Moon and the Incredible book of Hypnotism" series. Fun.
Martha is in bed now, but tomorrow I'll ask her and she will reel off an entire list of fabulous books for you to suggest. Watch this space.
Clare