This was just a decade after Augustine had written On the good of marriage, On holy virginity, and On the work of monks, meaning that he had then written about all the major styles of living. De bono viduitatis contains about 12,000 words. It was written as a letter, and Augustine considered it such. Because of this, it was not mentioned by Augustine in his Retractiones (“Retractions’), and Possidius mentions it in chapter 7 of his Indiculus (his list of Augustine's writings).
Augustine wrote De bono viduitatis in reply to a letter from Juliana, the widow of Olybrius. The late Olybrius was the eldest son of Anicia Faltonia Proba, who herself was the recipient of Augustine’s famous Letter 130 on the Lord’s Prayer (popularly called the “Letter to Proba.”) Proba, Juliana and Juliana’s daughter Demetrias had fled the sack of Rome in 410. They finally had settled in Carthage, where Juliana was enrolled as a consecrated widow in 412 and Demetrias accepted consecrated virginity in 413. Augustine composed a number of letters to these three women.
In De bono viduitatis Augustine implicitly warns these ladies of aristocratic birth not to be attracted to Pelagianism by warning them about its errors. The work has two sections, the first is instruction on widowhood (Chapters 2-15), and the second (Chapters 16 to the end) is exhortation. Most of the instruction is Augustine’s interpretation of the teaching about marriage and celibacy in Paul’s first Letter to the Corinthians. Echoing what Augustine had already stated a decade earlier in his treatise, De bono conjugali (“On the Good of Marriage”), Augustine repeated that Paul’s affirmation of the superiority of celibacy was not a disparagement of marriage. Even so, he here then adds that a consecrated widow such as Juliana to whom he is writing occupies a higher place among the members of Christ than does a married woman (De bono conjugali 3.4).
But, as well as the above, De bono viduitatis for the first time presents Augustine’s insistence on the unity of virgins, windows and married women within the unity of all Christians in the Church. Contrary to a common acceptance of his day, Augustine did not hold that the state of widowhood made a woman a "bride of Christ" such that a second marriage by her was in some way offensive to Christ. For Augustine, it was the church that was the metaphorical “bride of Christ”, and he was not inclined to use that metaphor of any individual person consecrated to Christ in virginity or widowhood.
In the second section of De bono viduitatis, Augustine exhorts Juliana to appreciate that the chastity she now vows is a gift from God, and that she should support her acceptance of that gift not by replacing the gap left in her life by the death of her husband with a love of wealth, but with reading, prayer, holy meditation, and by filling her heart with thanksgiving to God and with hope of the world to come (De bono viduitatis 21.26-27).
On the good of widowhood. Augustine's treatise De bono viduitatis online, as translated into English. http://www.newadvent.org/fathers/1311.htm AN2118