I said "What was that book you read?" Sounds right to me.
I wrote the same thing and it was wrong too
"What was that book you read" -sounds natural and should be accepted too.
Just reported it. Always report missing answers, they won't notice it here.
Why do you add 준 to 읽어? And not 읽은? At first I thought it was 읽어 주다 so what was the book you gave..?
"what book was that you read?" should be enough, no need 'it' i guess
Question, doesn't 무슨 mean what kind of book?