"What book does this student read?"
Translation:이 학생은 무슨 책을 읽습니까?
In Korean these sorts of questions are stuctured the same way as the answers themselves. I.e. a potential answer to this question is 이 학생이 저 책을 읽습니다. Notice how the word order is exactly the same in the corresponding question.
So instead of translating an English OSV question "what book does this student read?" directly try extracting the subject, object and verb from the corresponding SVO question "this student reads what book?"
You should get this:
Subject - this student
Verb - reads
Object - what book
Subject - 이 학생이
Verb - 읽습니까
Object - 무슨 책을
And stitch together using Korean SOV grammar:
이 학생이 무슨 책을 읽습니까?
Hopefully anyone confused finds this useful.
Use the topic marker (은/는) for generalizations and for contrasting/emphasizing something. The subject marker (이/가) is all about who is doing an action. There's some overlap but in general use the topic marker for talking with respect to someone/something and the subject marker for other cases where appropriate.
For better details, I recommend reading the tips and notes for the Basics 2 skill accessible from the mobile/desktop site (not from the app unfortunately).