The Nineteen Letters is fantastic but I don’t see how anyone, especially a teenager, can learn it without a guide. Any edition.
There’s Rabbi Tatz’s book for Thinking Teenagers. Forgot the full title. It may also be heavy but I will assume he writes in a somewhat accessible way.
There’s also a book put out by Feldheim by R’ Doron Korbleuth specifically for teenagers. He has a few good anthologies of general thought; you might enjoy the others too.
Thirty years ago JEP put out some wonderful books. They may not be sophisticated enough – maybe geared more for earlier teens – but they’re good reads. The hashkafa book is Lehavin ULehaskil.
Do any of your friends have teachers they can ask?
Any teachers in the CY?
Hatzlacha!