From the Feldheim website:
Of the many works of Rabbi Moshe Hayyim Luzzatto, Mesillat Yesharim stands out as his magnum opus. Ever since it was first published in 1740 in Amsterdam, it has enjoyed great renown and was eventually adopted as a basic text for ethical study. Throughout the long history of its publication, Mesillat Yesharim fell prey to many printers' errors. A breakthrough in its restoration occurred with the remarkable discovery by Ofeq Institute of a manuscript of an earlier version in the form of a dialogue, in the author's own hand. With this discovery and the aid of the first edition, Mesillat Yesharim was restored to its original state.
Over the last decade, Ofeq Institute has published both versions of Mesillat Yesharim, the Dialogue and the Thematic, in twin editions. For although the two versions share the same content, they each supplement elements missing in the other. Of pivotal importance are the added chapters at the beginning of the Dialogue Version. These shed light on the profound nature of the work as a whole. It has now become common practice to study the two versions together, for the Dialogue Version reveals the brilliance of Mesillat Yesharim for all who seek to deepen their understanding of it.
You can buy the book at the link above which includes both versions with translation.