In this book Augustin refutes the second letter 2319 which Petilianus wrote to him after having seen the first of Augustin’s earlier books. This letter had been full of violent language; and Augustin rather shows that the arguments of Petilianus had been deficient and irrelevant, than brings forward arguments in support of his own statements.
Possidius, in the third chapter of his Indiculus, designates this third book as "One book against the second letter of the same." Cp. Aug. Retractt. Bk. II. c. xxv.