About the author, I found this in the intro of the Book in the New American Bible:
The author of the book was a teacher of popular wisdom (Eccl 12:9). Qoheleth was obviously only his literary name. Because he is called "David's son, king in Jerusalem," it was commonly thought that he was King Solomon. Such personation, however, was but a literary device to lend greater dignity and authority to the book-a circumstance which does not in any way impugn its inspired character. The Epilogue seems to have been written by an editor, probably a disciple of Qoheleth. The entire work differs considerably in language and style from earlier books of the Old Testament. It reflects a late period of Hebrew, and was probably written about three centuries before Christ.
2cor 7:1 Therefore, having these promises, most beloved, let us cleanse ourselves from all defilement of the flesh and of the spirit, perfecting sanctification in the fear of God.