My granddaughter is a vorocious reader at 12 years old and reads at a higher level then normal for her age.  She has been reading The Chronicles of Narnia and loves the series.  Has read Little House on the Prairie and Nancy Drew series also.  Can you give me some suggestions of new authors? 

Nancy, glad to hear your granddaughter loves to read!
How about Harry Potter (JK Rowling)? Judy Blume?

I remember reading Nancy Drew and Little House on the Prarie. Around the same time I read Sweet Valley High/Twins, and a YA author named Christopher Pike.

She might like the Anne of Green Gables series.  Trixie Belden is a fun series though not real easy to find.

Nancy

Here are a few more ideas. 

 The Secret School - Avi

 A Year Down Yonder - Richard Peck

 The Misadventures of Maude March - Couloumbis

 Lunch Money - Clements

 Esperanza Rising – Pam Munoz Ryan

 Number the Stars – Lois Lowry

 The Watson’s Go To Birmingham 1963 – Paul Curtis

Out of the Dust - Karen Hesse

Letters from Rifka  – Karen Hesse

 The Hundred Dresses – Estes

 In the Year of the Boar and Jackie Robinson – Bette Bao Lord

 The View from Saturday – Konigsburg

 The Kite Fighters – Park

 Snow Treasure – McSwigan

 Journey to Ellis Island – Bierman

 Immigrant Kids – Freedman

 Rodzina - Karen Cushman

 Princess Academy - Shannon Hale

 Just Ella - Haddix

 Listening for Lions – Gloria Whelan

 Once on This Island - Gloria Whelan

 The Westing Game – Ellen Raskin

  Blood on the River: Jamestown 1607 - Elisa Carbone

 The Sacrifice – Duble

Some of my DD's fav authors are Gloria Whelan, Karen Hesse, Karen Cushman, Ann Rinaldi, and Avi.

I have two series on my shelf that my daughter enjoyed:  Left Behind:  The Kids and Pacific Cascades. 

It's great your granddaughter loves reading!

Vicki

This is one of my favorite ages.  If she likes Narnia, she would most likely love the Chronicles of Prydain series by Lloyd Alexander, I would also give the Redwall series by Brian Jaques a shot.  I strongly recommend both of them, although the Redwall books do tend to be somewhat long (although they are well-written enough that they hold my 8 year old nephew's interest).  Also keep in mind with the Redwall series that there are 20 books in the series (each can be read independently) and Jaques seems to have no inclination to stop anytime soon.

There's a good chance she's already read some, but I always recommend Roald Dahl for this age.  If she hasn't ready any of his books, Matilda would be a good place to start.

I also recently read the first two books of the Tunnels series, and thought they were really good.