"The orator reads many books."
Translation:Orator multos libros legit.
In the plural multi libri can be either the nominative (the subject, the doer of the sentence) or the vocative (direct address, talking to the books). The many books are neither doing the reading or being talked to so we cannot use multi libri.
We have to use multos libros because that is the accusative plural, the case that is used when something is the direct object, the thing the action is being done upon. The books are being read, the reading is done too them, so we use the accusative.