Mike Aquilina alerts us to a freebie:
The earliest Christian commentary on the Song of Songs is, at long last, available in English. Yancy Smith embedded a translation in his doctoral dissertation, Hippolytus’ Commentary on the Song of Songs in Social and Critical Context, which is available free online. You can also get to it by going to the TCU website, lib.tcu.edu. Then input either the author name or title.