Even though the Bible was written by a diverse group of writers across a long time span, the primary author of the Bible was the Holy Spirit. There is an essential underlying unity to it. How it interacts with itself is important to understanding this second layer of meaning.
The first and primary way this occurs is discussed in the next section about Ruth and Judges. These books speak indirectly to each other. When we read between the lines, we get this.
The second and more dramatic way this happens is when we see how the New Testament fulfills the Old Testament, showing us how to read and see Old Testament passages in light of their New Testament fulfillment. A clear example of this is seen in the parallel passages of Exodus 24:8 and Luke 22:20.
"Moses took the blood and threw it on the people and said, 'Behold the blood of the covenant that the Lord has made with you in accordance with all these words.'" (Ex 24:8)
"Likewise the cup after they had eaten, saying, 'This cup that is poured out for you is the new covenant in my blood.'" (Lk 22:20)
We read the sacrifices of the Old Testament with the light of the ultimate sacrifice shown to us in the New Testament. This follows the Reformation principle of sola Scriptura that Scripture interprets Scripture.
This principle also applies to less clear and more difficult passages like Matthew's use of Hosea 11:1 in Matthew 2:15: "Out of Egypt I called my son." The primary question we should ask ourselves is 'How is the New Testament interpretation of the Old Testament continuous with its original intent and meaning?' If we see it as discontinuous, then we know we are interpreting some part or all of these passages incorrectly.
This can lead into more technical discussions about typology and the like, but without going into any of that, suffice it to say that the New Testament's usage of the Old Testament shows us how to properly understand it. The Old Testament contains a real grasp of God's messianic plan even if that is otherwise murky to its original writers.
Moses and Hosea trusted that God would faithfully fulfill his promises even though Jesus would not be born until well into their future and they could not specify an exact description of that. This parallels our own faith and hope that Jesus will one day somehow reconcile all things to himself and restore all of creation. We cannot specify an exact description of this, and yet we know enough to trust in his promises and believe that salvation comes only by faith in him.
So, the primary way we get at the covenantal layer is by asking how the text we are currently reading interacts with and interprets the other biblical texts we have already read. This is an unending cycle and presupposes that we are actively and continually reading the Bible. In order for the biblical story to become our story, we must read enough of it frequently enough that it fills our memory with its stories.
Just as foreshadowing often requires multiple readings before you start to get it, so also will your ability to read implicitly improve with multiple readings of the Bible. Implicit reading is the reading between the lines reading that enables you to see what was always embedded within the story but is easily overlooked during a casual reading. This has nothing to do with hidden or secret meanings or any of that nonsense. Instead, this is about seeing the stuff that the narrator and his initial or implied audience would have simply assumed to be present in the story. In other words, when you read between the lines, you are looking for what the narrator is implying to his audience without directly saying so.
The story of Ruth illustrates this. The central theme of this story is about God's loving kindness to the three primary characters of this story. Once we are very familiar with biblical geography, history, and customs, we will then begin to hear what else it is telling us. This is decidedly not some sort of secret message. In fact, this was the obvious stuff that was so obvious to its original audience it did not need to be said directly.
To get what Ruth says implicitly, you need to know that this story's setting is the same as that of the book of Judges. That is explicitly mentioned in Ruth 1:1. You will need to remember from your prior readings of Judges that during this time Israel had widespread idolatry, deep systemic social injustice, sexual immorality, and just lots and lots of unfaithfulness to God.
Against this backdrop setting, we read in Ruth that Ruth was a convert to faith in the God of Israel. The text never says this directly, but we pick it up in her opening dialogue. She swears an oath by Israel's God (1:17). We then read that Boaz was righteous. Again, this is not explicitly written, but we see it through his actions and words. He is faithful to the commands of Leviticus about gleaning (Lev 19:9-10) and the law of redemption (Lev 25:23-24), for example.
We also learn that this foreign convert woman Ruth is part of the ancestry of King David and therefore also of Jesus. In fact, from the genealogy of chapter 4 we see that Ruth is the great grandmother of King David. This is important in both the immediate and covenantal layers. These genealogies tell us that David and Jesus' ancestry is not about some sort of racial or moral purity. They are about God's faithfulness to his promises.
We can also read between the lines to learn that Bethlehem was an exceptionally faithful little town. Again, this is never explicitly stated, but the faithfulness to God of almost all of the characters of this story shows us this. They are all from Bethlehem. Consider Naomi, Boaz, the town elders, and even the ordinary people whose greetings in 2:4 show their faithfulness to God. Implicitly and especially against the backdrop setting of Judges, we are told that Bethlehem is a special place. This is part of both the immediate and covenantal layers of meaning in Ruth.