I think there's mathematical methods you can use to calculate the reading level of a book. Always have trouble finding them online though. It'd be nice if there were a tool that could do the calculations for you.
Here's a site where you can request a Lexile level calculator tool.
https://lexile.com/analyzer/
Never used that particular calculation / calculator, so I don't know if that'll help you.
There's another site I found titled readabilityformulas (with a com for the address). That one seems to list a few methods for calculating US grade levels. I think I used Smog?