We had a problem when my son was that age finding books that piqued his interest without the boy/girl relationships or adult themes he was not yet ready for or violence. It was hard for me to find books for advanced readers in the preteen years.
Some authors my son loved at that age were Brian Jacques (esp the Redwall series,) Harry Turtledove (he writes fantasy and historical fiction,) David Gemmel, and RA Salvatore. Gary Paulsen writes for that age group ( child versus conquer the elements and nature type books) but my son read those in 4th grade so he might have already read them.
The staff at Barnes and Noble was always pretty familiar with their books so could tell me if there wsa too much adult content in them. Our local librarians were never any help.