I think "tweens" are a hard reading crowd. They are desperate to read more sophisticated books but at the same time the content just isn't appropriate. I tend to err on the more conservative side. There is so much wonderful literature out there, keep them reading the good stuff as long as you can. But at the same time they want more. Here is a list in alphabetical order by author of some great "tween" reads. Many times once an author writes for this age group they tend to write more books so you will see multiple entries by the same author or I just didn't put all the titles in meaning you can't go wrong with any of them. I know I probably haven't even gotten them all, but this is a start and I'll continue to revise the list.
The Red Blazer Girls by Michael Beil
The Penderwicks by Jeanne Birdsall
The Penderwicks on Gardam Street by Jeanne Birdsall
The Penderwicks at Point Mouette by Jeanne Birdsall
Are you There God? It's me, Margaret by Judy Blume
Here's to You, Rachel Robinson by Judy Blume
Just as Long as We're Together by Judy Blume
Starring Sally J. Friedman as Herself by Judy Blume
Deenie by Judy BlumeThe Face on the Milk Carton by Caroline Cooney
Walk Two Moons by Sharon Creech
Weedflower by Sharon Creech
The London Eye Mystery by Siobhan Dowd
The Graveyard Book by Neil Gaiman
Eleven By Patricia Geiff
Pictures of Hollis Woods by Patricia Geiff
Princess Academy by Shannon Hale
Book of 1000 Days by Shannon Hale
Goose Girls by Shannon Hale (and any other of her books)
Ida B by Katheine Hannigan
Emmy and the Incredible Shrinking Rat by Lynne Jonell
Emmy and the Home for Troubled Girls by Lynne Jonell
The View from Saturday by E.L. Konigsburg
Schooled by Gordon Korman
Savvy by Ingrid Law
Scumble by Ingrid Law
Ever by Gail Carson Levine
Fairest By Gail Carson Levine
Ella Enchanted by Gail Carson Levine (and any other of her books)
Rules by Cynthia Lord
Miracle on 49th Street by Mike Lupica
11 Birthdays by Wendy Mass
Kiki Strike: Inside the Shadow City by Kirsten Miller
Kiki Strike: The Empress Tomby by Kirsten Miller
Princess Ben by Catherine Murdock
Eleven by Lauren Myracle
Twelve by Lauren Myracle
Thirteen by Lauren Myracle
Love Ya Bunches by Lauren Myracle
A Long Way from Chicago by Richard Peck
Esperanza Rising by Pam Ryan
The Wednesday Wars by Gary Schmidt
Star Girl by Jerry Spinelli
When You Reach Me by Rebecca Stead
The Mysterious Benedict Society (three books in the series)by Trenton Lee Stewart
A Crooked Kind of Perfect by Linda Urban
Flipped by Wendelin Van Draanen
Runaway by Wendelin Van Draanen
So B. It by Sarah Weeks
The Incorrigible Children of Ashton Place: Book One The Mysterious Howling by Maryrose Wood
The Incorrigible Children of Ashton Place: Book Two The Hidden Gallery by Maryrose Wood