Fraunceys Petrak, the lauriat poete,
Highte this clerk, whos rethorike sweete
Enlumyned al Ytaille of poetrie
(Clerk's Prologue, IV.31-33)
Francis Petrarch had an enormous influence on English literature, beginning in the sixteenth century -- the fact that we customarily Anglicize his name, Francesco Petrarca, into "Francis Petrarch" shows how deeply embedded his works are in the English poetic tradition. His poems shaped much of Elizabethan lyric poetry, and Shakespeare's sonnets could not exist without Petrarch's previous sonnets and canzone.
For further information on Petrarch, see his Wikipedia entry (to be read with the skepticism one brings to any secondary source, whether digital or printed).
Chaucer was the first English writer to know these poems, and -- centuries before Petrarch's work entered the mainstream of English literature -- he drew on Petrarch for Troilus' song in Troilus and Criseyda (Bk I, 400-420) -- "If no love is, O god, what fele I so?" This is an adaptation (and sometimes close translation) of Petrarch's Sonnet 88, "S'amor non è."
For a text of this sonnet, see:
Petrarch was as celebrated in his time for his Latin works as for his Italian (when Chaucer calls him the "laureat poete" he refers to the Latin works). Most, like his ambitious but finally unsuccessful epic Africa (celebrating Scipio Africanus), are now almost forgotten, but his often charming Latin letters (such as the letter to posterity) are still worth reading. See
H. Robinson and E.H. Rolfe, Petrarch: The First Modern Scholar and Man of Letters. New York. 1899 [Widener Ital 7140.23.4]. Available on Google Books for Harvard users.
(These are not intimate personal letters but rather exercises in a literary genre, meant for publication).
Chaucer drew upon one of Petrarch's letters to Boccaccio, Petrarch's good friend, for the direct source of The Clerk's Tale. The letter consists of a kind of commentary, an introduction to the tale, and the tale itself, rendering Boccaccio's Italian into elegant Latin prose. The introduction is interesting for Petrarch's comments on this difficult story:
Chaucer closely follows Petrarch's Latin version of the story: