Recommended edition: De vulgari eloquentia, tr. Steven Botterill (Cambridge: Cambridge UP, 1996).

From the publisher:
De vulgari eloquentia, written by Dante in the early years of the fourteenth century, is the only known work of medieval literary theory to have been produced by a practising poet, and the first to assert the intrinsic superiority of living, vernacular languages over Latin. Steven Botterill here offers the only Latin-English parallel-text edition of an important early work by the author of the Divine Comedy, and the only English translation with thorough and up-to-date notes and introductory material.

Online:
Amazon (recommended version)