There are three papers in this one volume to which I particularly want to draw attention: "God's Grace and Man's Need," "Two Warnings and an Example," and "Wilderness Grace."
#8 "God's Grace and Man's Need" (available from STEM Publishing)
The first might be Darby's best gospel message. It's short, gets right to the point, and makes the case very simply that God offers justification freely to man, because man has nothing to offer God. It was always God's plan not to demand from man, but to offer forgiveness freely, based on the One Man's offering Himself without spot to God.
To turn for a moment to the use man made of the law, in contrast with God's purpose in it: God used it, as we have seen, that the offence might abound - that sin might appear exceeding sinful. Man set about to make himself righteous by the very thing by which God was proving him a sinner, and sin exceedingly sinful. This you are doing, if you are seeking to satisfy the demands of God's righteousness by your own ways. Man seeks to save himself by the righteousness of the law; but God's use was not that, for He never thought of saving any but by Jesus.
#7 "Two Warnings and an Example" (available from STEM Publishing)
This is a brief study of three main characters in the Garden of Gethsemane: the Lord Jesus, Peter, and Judas Iscariote. It's well worth a read, as it contrasts the unregenerate, the carnal Christian, and the Perfect Man in under stress.
When Christ was praying, Peter was sleeping; when Christ was submitting as a lamb led to the slaughter, Peter was fighting; when Christ was confessing in suffering, Peter was denying Him with cursing and swearing. This is just the flesh: sleeping when it ought to be waking; in energy when it ought to be still; and then denying the Lord when the time of trial comes.
#6 "Wilderness Grace" (available from STEM Publishing)
This is an interesting study: it contrasts God's dealings with Israel from the time they crossed the Red Sea until Sinai, with His dealings with them once they received the Law.
To bring into the wilderness, and not at once into Canaan. Being in the wilderness implies all sorts of trials. It may seem strange to sight, that they who had just been singing the song of triumph and deliverance (chap 15) should be allowed to be three days in the wilderness without water; and then, when they came to water, should find it so bitter that they could not drink of it. But God permits these trials, in order that we may see our own need and prove His faithfulness. From the Red Sea to Sinai Israel proved the grace which belongs to us now.
If you are at all interested in reading Darby, you really need to start with Volume 12 of Collected Writings. It gives perspective to everything else he wrote, as well as being the most readable and approachable of his writings.