Tolkien and J.K Rowling literature is pretty simple
Try reading teen-oriented thrillers/detective dramas too, that should do the trick.
The Hobbit definitely, but not the Lord of the Rings or the Silmarillion. Have you even read them?
I remember the first time I read LOTR in english. I learned a lot of new words that I haven't seen that often since.
Definitely not easy.
OP, what's your mother language?
"To Kill a Mockingbird" by Harper Lee or anything by George Orwell (like 1984 or Animal Farm) both of those are taught like middle and early high school years so theyre good for strengthening english vocab.