"She reads him a book."
Translation:O ona bir kitap okur.
"Onu" is in the accusative case (direct objects). "Ona" is in the dative case (destinations or indirect objects).
In this sentence, "ona" is the person to whom she's reading: the indirect object. "Onu" would be used for the thing she's reading. (For instance, if you wanted to say "she reads it," you'd say, "onu okur.")