My dd is 12. Her favourite author is Alice Oseman and the heartstopper series. I read these aswell as there is some difficult subjects but I made sure she could talk to me if she didn't understand or found it upsetting. However I think it may have helped her as she has a few friends struggling with the topics in the books.
She also really enjoys LGBT love stories. Her favourite is girl heart girl but she has many more too!
She really enjoys a graphic novel - sad ghost club, girl from the sea, bloom are her favourites.
She has started reading a good girls guide to murder and is enjoying it so we are going to explore more crime/mystery books.
She has got hunger games and maze runner to read as she loved the films. So may be good to see if there's any films/series she likes that were books first.
What we did was go to cheap charity shops and pick up anything that sounds interesting to her, no pressure to finish and she gets to try lots of genres and authors. We can then donate/sell the books after. We have some where books are 3 for £2 and one where they do 8 for £1.
My eldest dd however only reads manga and costs me a fortune! She started around 12. But be aware that they are age rated for a good reason, some are not suitable for children! Check the back for the age and if it's sealed its a definite no!
On that note make sure she does NOT read Wicked even if she likes the film/show. The book is not suitable for children at all despite the new film cover for the current cover of the book.