You don't need to say "okuduğun" because it's already clear that we read the book. Okuduğun kitap = The book which you read We don't necessarily need to mention that we read the book. Because what can we do with books other than reading them? 😅 If you want to add the verb "oku-mak" you can say the second sentence I wrote 🥰🙏 But as a native speaker I would say "Hangi kitabı okuduktan sonra hayatın çok değişti?" They both correct.