Update: In addition to the commentaries below, Logos also is offering a free book from Zondervan, The Promise-Plan of God by Walter Kaiser, which is a Biblical Theology of the Old and New Testaments.
The Logos “Free Book of the Month” is a commentary on Amos by Gary V. Smith from the Mentor Commentary series.
Be sure to head to the site and pick it up—it’s a good one from what I hear! And if that’s not quite enough for you, they also have the commentary on Joel & Obadiah on sale for $1.99 (that’s 91% off!).