The guardian? That's pretty good for learning grammar but might contain some propaganda. How about a non-fiction book? They're always really good and teach you more than vocab. Something about engineering or science (except evolution) at a very basic level?
15 years old is old enough to not be influenced by Harry Potter into becoming a kafir. People need to treat youngsters with a little more trust and not underestimate them so much. I read and enjoyed the Narnia books- Christian "propaganda" if there ever was any- before my teens. I did not become Christian, A teenager is mature enough to distinguish between fantasy and reality. If they have been given a firm grounding in aqeedah like they should have, then this stuff should not affect them, unless they are just weak-minded.
I think a more valid argument against fiction is that it's a waste of time that could be better spent reading nonfiction like history or science, which a 15 year old is definitely old enough to do. But there are a lot worse things kids could be doing for recreation than reading Nancy Drew.
One somewhat less-known novel I recommend "The Phantom Tollbooth" by Norton Juster. One of my favorite books, with a good message about education and appropriate for all ages.
Allah knows best. Other good Islamic reads I think can include:
~ The Ideal Muslim / The Ideal Muslimah
~ Don't Be Sad
~ Enjoy Your Life
~ any Islamic book about the male and female companions, especially the wives, of the Messenger Muhammad (SallAllahu 'alaihi wa sallam)
~ the English translation of the Holy Qur'an (if she can read the whole Qur'an in its English translation, along with a dictionary in case she doesn't understand a certain word, I think she'll insha-Allah be able to improve in her English grammar and vocabulary)
Allah knows best
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