DD10 has hit the ceiling of the MAP test and is reading at a high school level. The challenge is to find books that are at a more advanced reading level, that don't bring in too much sex or violence. Books from a century or two ago are generally safer bets, since society censored more back then.
Here we can assess the kid-safe:
https://www.commonsensemedia.org/Here we can assess the grade-level or lexile:
http://www.scholastic.com/bookwizard/I just wish these two databases were merged. I still have not found a definitive solution to this problem. Its exacerbated by how fast gifted children consume books.