I think most of the books I loved as a young teenager have been mentioned already, but here’s a few more (apologies if I’ve missed people mentioning them already):
which witch? by Eva Ibbotson, it’s a really sweet story and I still love it
Would thoroughly recommend The Changeover by Margaret Mahy, it is beautifully written and I really enjoyed re-reading it recently.
The Peacock Spring and The Greengage Summer, by Rumer Godden
The Cats of Seroster, by Robert Westall, I haven’t reread that recently but do remember liking it a lot at the time.
One more river by Lynne Reid Banks - it’s not on kindle and I think is out of print, so I’ve not managed to reread that, but I reeeally want to!