"Neden o kitabı siz yazmıyorsunuz?"
Translation:Why are you not writing that book?
Yep! Subject pronouns are always optional unless you have two contrasting subjects in one sentence. :)
And why does the subject come after the object here? Anything gets emphasised this way?
In this case, the subject "siz" is getting stress. In general, the things closest to the verb in Turkish are stressed. :)