I started Richard Nelson's commentary on the Book of Joshua. I have three items.
1. Nelson does not believe that the Book of Joshua presents an accurate account of the Conquest, for the same reasons that many biblical scholars reject the historicity of the Conquest (see here). But he does maintain that the Book of Joshua contains what people thought happened to their ancestors---which contradicts the usual platitude that says that the biblical authors did not intend to write history, but something else entirely (e.g., an allegory, though I rarely hear what the alleged allegory is supposed to represent). In the early Iron Age, there were two groups of people, according to Nelson:
"In the lowlands was the established, elitist culture of the city-states with their kings and chariots. At the same time, an alternate social system was developing in the highlands. This was an egalitarian, rural village culture, without the social stratification that comes with being organized as a state. It depended on a largely self-contained economy based on farming and herding." (Page 4).
This community in the highlands was Israel. According to Nelson, the people who became Israelites may have been pastoralists who settled down and pursued agriculture, or perhaps they came from the Canaanite city-states. But their material culture overlaps with that of Late Bronze Canaan---and even the "epigraphic finds utilize the Canaanite alphabet" (page 3)---so they most likely were indigenous to Canaan, rather than people who came from the outside, as the Book of Joshua depicts.
As Judges 5 (the Song of Deborah) indicates, there was conflict between the Israelites in the highlands and the Canaanites in the lowlands. For Nelson, the Book of Joshua served to unify the Israelites and give them an identity. It allowed them to celebrate YHWH as their Divine Warrior and to lay claim to the land of Canaan, even as outsiders, for "It is common for traditions of national origin to speak of immigration from another place, as Israel itself was aware (Genesis 10; Amos 9:7" (page 5). The Conquest in the Book of Joshua also reflects ancient Israel's xenophobia, and the Book of Joshua long resonated with Israelites because their possession of the land was continually at risk---due to Canaanites and foreigners. During the time of the monarchy, the Book of Joshua provided Israel with a "unified narrative of origins" (page 5). And the Book of Joshua continued to be relevant even during the exile, as it reminded Israel that God gave her the gift of land, which she forfeited through her disobedience.
According to Nelson, the Book of Joshua also served an etiological function, in that it offered a story that accounted for aspects of ancient Israel's life: the presence of a foreign Rahab clan, ruins of Canaanite cities, a tomb of Achan, etc. For Nelson, the people who created the Book of Joshua drew from traditions uncritically to produce a narrative.
2. On page 29, Nelson argues that the Deuteronomistic contribution to Joshua 1 portrays Joshua using royal concepts, perhaps "to create parallels between the figures of Joshua and Josiah." YHWH's charge in Joshua 1:1-9 resembles David's charge to Solomon in I Kings 2:2-4. Joshua is also to meditate on the law of the LORD, like the ideal Deuteronomic king (Deuteronomy 17:18-19), and the uncompromising obedience to him by the eastern tribes, his responsibility for the military success of Israel, and the legitimation of his authority by the LORD's presence also echo biblical stories about kings. The idea here may be that the Deuteronomist wanted to promote Josiah, and so he added details to the respected story of Joshua in order to make Joshua into a Josiah-like figure.
3. Nelson's textual critical model appears to be to look at the MT and the LXX and to eliminate additions that each seems to have made. There are things in the LXX that are not in the MT, and vice versa, and Nelson may presume that the simplest text was the early one, whether it is in the LXX or the MT.
An example of Nelson's methodology occurs in his exegesis of Joshua 5. Joshua 5:11 affirms that the Israelites ate unleavened cakes and parched corn, but the MT ties that to the Passover, whereas the LXX does not. Nelson treats the part about the Passover as an addition. Joshua 5 still has a Passover, for v 10 mentions it, even in the LXX. But the connection between the Passover and the unleavened bread is something that the MT adds to Joshua 5, according to Nelson.