I tend to agree that forcing a child to read so much higher than age level can have negative consequences. That is the reason I focused on books written and intended for kids or young adults that still had high lexile levels (they DO exist). The lexile level is really an indicator of the complexity of vocabulary and sentence structure a reader will encounter--NOT the complexity or level of the plot itself.
On the flip side, I am not a fan of telling children they may NOT read above grade level. That can be just as frustrating to a strong reader.
OP--did you see the post I made while you were posting, with suggestions of juvenile books for most of those categories? It gives you more to look for at half price books (and don't forget the library!

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