Heidegger's Bible Handbook: Ecclesiastes: Inscription and Authorship
1. The Inscription. Solomon was the author of the book, not the collector.
The author of this book is called קֺהֶ֣לֶת בֶּן־דָּוִ֔ד מֶ֖לֶךְ בִּירוּשָׁלִָֽם׃, Qoheleth, the son of David, King in Jerusalem, Ecclesiastes 1:1. It is ἀνεξαγώνιον, beyond question, that King Solomon is indicated. For, although perhaps one, even Grotius, who makes another the author of the collected words of Solomon, would call this into question, relying upon trifling argument, that in this book are extant many vocabulary words, which are found in no other place than in Daniel, Ezra, and the Chaldeans (why may not these words be recalled to the root of the Hebrew tongue, the mother of the rest, especially of the Chaldean?): nevertheless, that it was written by Solomon is manifestly argued, both by the very eloquently expressed inscription; and by the frequent use of the first person, I built, I understood, etc., which a collector would not attribute to himself; and by the elegant order, which other rhapsodist had not so observed; and what he narrates, that he built houses, gardens, pools, etc.; and, finally, by unanimous consent of the Hebrews, Greeks, and Latins.