HighIQ, the answer may depend upon whether one is looking at a reading level measurement, such as as a Lexile Level, or whether one is considering the topic, concepts, genre, emotional maturity of the books.

Some links which may be of interest:

- Lexile levels - https://lexile.com/parents-students/measuring-growth-lexile-measures/college-and-career-trajectory/

- The Recommended Resources forum (here on Davidson Gifted BB) has crowd-sourced lists of books by age. These lists are created by forum members, including parents, teachers, gifted individuals. Feel free to add to them.

This particular list is for ages 9-12 -
http://giftedissues.davidsongifted....recommendations_age_9_12.html#Post193027

This list is for ages 13+ -
http://giftedissues.davidsongifted....k_recommendations_age_13.html#Post193028

- This list contains books for adults -
http://giftedissues.davidsongifted..../198136/books_for_adults.html#Post198136