The ending of the book brings an exhortation to avoid ill-will, and to seek prudence, faith, and the other virtues.
152. My sons, avoid wicked men, guard against the envious. There is this difference between a wicked and an envious man: the wicked man is delighted at his own good fortune, but the envious is tortured at the thought of another's. The former loves evil, the latter hates good. So he is almost more bearable who desires good for himself alone, than he who desires evil for all.
153. My sons, think before you act, and when you have thought long then do what you consider right. When the opportunity of a praiseworthy death is given let it be seized at once. Glory that is put off flies away and is not easily laid hold of again.
154. Love faith. For by his devotion and faith Josiah won great love for himself from his enemies. For he celebrated the Lord's passover when he was eighteen years old, as no one had done it before him. As then in zeal he was superior to those who went before him, so do ye, my sons, show zeal for God. Let zeal for God search you through, and devour you, so that each one of you may say: “The zeal of your house has eaten me up.” An apostle of Christ was called the zealot. But why do I speak of an apostle? The Lord Himself said: “The zeal of your house has eaten Me up.” Let it then be real zeal for God, not mean earthy zeal, for that causes jealousy.
155. Let there be peace among you, which passes all understanding. Love one another. Nothing is sweeter than charity, nothing more blessed than peace. You yourselves know that I have ever loved you and do now love you above all others. As the children of one father you have become united under the bond of brotherly affection.
156. Whatsoever is good, that hold fast; and the God of peace and love be with you in the Lord Jesus, to Whom be honour and glory, dominion and might, together with the Holy Spirit, for ever and ever. Amen.
Source: On the Duties of the Clergy (New Advent)