I'm sorry I don't quite understand, why is "hangi kitaplar okursun?" wrong ?, I understand that when saying "Hangi kitabı okuyorsun?", you are asking about a certain book (maybe among a collection or a set of books that you know about or have), but why would you add "ı" if you are asking about books in general ?
Which book are you reading?
"Turkish grammar in practice" - A self study reference & practice book. Author - Yusuf Buz. Millions of learners puts the MOD's finite time in answering learner questions under pressure. I can relax & read this book @ home in peace. Please note: The tips & notes provided by the MOD's are invaluable & this book supplements learning. "Size alışmak kolay gelsin." Teşekkürler.