Each month, Logos gives away a free book, and makes another available for US$2.
For October 2016, the freebie is Craig Keener’s 2009 commentary on Romans, from the New Covenant Commentary Series. Keener is a good NT scholar, and this commentary puts you in touch with current scholarship. Here’s a sample from the Introduction (pages 15-16):
Paul certainly also summarizes his gospel, as many scholars point out; nevertheless, as we have noted above, he is not simply giving a random audience a random overview of it. It has practical implications for their situation, and the fact that half of the explicit biblical citations found in all his writings appear in this one letter suggests that this presentation of the gospel is directed toward a situation concerned with the status of Gentile believers vis-à-vis the law (likely a major point of division in a conflict between Jewish and Gentile factions).
The $2 commentary is Gordon Fee’s 2011 commentary on Revelation from the same series. It’s important to understand what kind of literature you’re reading as you approach the Book of Revelation, and Fee summarizes this issue succinctly (page xii):
What one must understand before reading John’s Revelation is that he has purposely set out to write something that has not been done before, something that he sets up his readers to understand at the very beginning. Thus in 1:1 he identifies what he is about to write as an apocalypse, translated “revelation” in the NIV, which in 1:3 he refers to as a prophecy. But in the next two verses he begins again with all the formal aspects of an ancient letter. So the reader is given these three different pieces of information at the outset. What is unique about John’s Apocalypse is the fine blending of each of these three kinds of literature—apocalypse, prophecy, letter—into a single whole piece.
Update (2016-10-04): A. J. Maas 1892 work, A Day in the Temple, is also free this month.