Please forgive my ham-handed translation skills, but I thought this article from La Repubblica was pretty nifty.

Dating from the 12th century, a handbook for letter-writing is uncovered at the Biblioteca Capitolare di Verona
Anticipating the famous correspondence between Abelard and Heloise, a cleric taught the words to use

“Cosí si scrive una lettera d’amore”: The first manual from the Middle Ages
The manuscript Modi dictaminum, in Latin, gives advice to women

The oldest Italian manuscript on how to write love letters dates back to the Middle Ages. Scholars from the University of Siena have traced the unpublished texts of ancient rhetoric to Italian and Spanish libraries. According to the researchers, it is the first manual for letter-writing to come to us, in which they also teach how to write love letters. The text, in Latin and on parchment, is from the second half of the 12th century. The author is a certain Guido, a cleric perhaps from Casentino, the zone on the border between Tuscany and Emilia.

The advice ranges from how to write the salutation of the love-letter to how to conclude it. But also as a wife must write to her husband, a lover to his beloved. It recommends always to praise the beauty and qualities of the recipient using comparisons to famous couples (Paris and Helen, Pyramus and Thisbe), similarities to precious stones or by sending “as many greetings as there are fish in the sea,” or “the flowers brought by the summer.” You must use phrases indicating the inability to describe such a large feeling (“How deeply I love you I cannot express with words, even if all the members of my body could talk”). When the sender has to tell the recipient something, he can use apostrophic expressions such as “your beauty knows,” “your sweetness knows,” “it is known to your nobility.” The distance from the subject of one’s love or the memory of the happy moments spent together is already assumed in the characteristics of lovesickness: “the mind is less,” “the soul cannot stand much joy.” Then there are passages which refer explicitly to physical love: it speaks of hugs, kisses, desire, of sweet things to do together.

The rhetoric teacher teaches his students how to write letters — not just of love but between parents and children, masters and disciples, lords of the laity and of the church, how to write to the Pope or the Emperor — taking portions of letters and copying them into the manual as examples. “According to our studies,” says Francesco Stella, professor of medieval Latin literature of the faculty at Arezzo (University of Siena) and coordinator of the research, “we are in front of the first correspondence manual with one chapter, the fourth, devoted to love letters. The teacher, among other things, gives advice to women writing, confirming the existence of a female audience of secular literate women in the Middle Ages.”

The news does not end there. “Many letters,” Stella continues, “are about Guido, patrons of part of Northern Tuscany, and of Emilia-Romagna, powerful feudal lords of Mathilda of Canossa and Frederick I. In particular, there is one which we suspect was the oldest example of a love letter from the Middle Ages.” The uncertainties depend on the fact that the original is missing, that there are only a few lines in Latin copies on parchment by the cleric Guido in the book Modi dictaminum kept in Verona. “To Imilde, dear wife,” it begins. A husband that we know only by the initial G. writes that while his wife is away, “I want you to know that by the grace of God I am in Pisa and I am well and I sold all the goods … your love, my sweetest friend, knows that for the perfume of your love does not refuse to cross the mountains or swim the seas.” The clues that suggest this as the oldest love letter that has reached us are based on the fact that most of the letter examples cited by the author of the manual refer to the archives that Guido drew from. G. who wrote to his wife Imilde could possibly have been Guido II. “We know,” says Elisabetta Bartoli, PhD student at the Faculty of Arts who is currently preparing the critical edition of the Modi dictaminum, “that Guido II and Imilde made a donation to a church in Casentino in 1017 and from another document that in 1029 Imilde is already dead. If it is so, we are a century before the most famous love letters, beginning with Abelard and Heloise.”

Some additional links on letter-writing in the Middle Ages and Renaissance are available at moas.atlantia.sca.org/wsnlinks/index.php?action=displaycat&catid=779.