“Qaraʾtu l-kitāba” names the object. In “qaraʾtu,” the object is unexpressed; context determines whether a particular object is understood or whether the purpose is to state the activity itself.
After “Did you read the book?”, the reply “Yes, I read” implies that book. But in “Anā aqraʾu wa-aktubu,” the speaker may be describing general capacities without intending any specific omitted objects. This distinguishes ellipsis of an identifiable object from using a transitive verb as though it were intransitive.