I have three items from my reading of Brian Peckham's History and Prophecy:
1. On page 345, Peckham states the following:
"The plagues [of the Priestly writer] have to do with portents on land and sea and in the sky, affect humans and animals, and are initiated by the magical creation of sea serpents (tannin, Exod 7:9, 10, 12; cf. Gen 1:21). The Deuteronomistic Historian added other plagues that affected the Pharaoh's court, estates, and servants, but noted the cosmological significance of the Priestly plagues by replacing the primordial process of separation (bdl, Gen 1:4, 6, 7, 14, 18) with the process of differentiation between Israel and Egypt (plh, Exod 8:18; 9:4; 11:7)."
There are two things in this quote that interest me. First of all, many English translations render tanin as "whale" in Genesis 1:21, but as "serpent" in Exodus 7:9-10, 12. Exactly what did God turn Moses' staff into? Second, Peckham says that DtrH adds specific plagues to the Exodus narrative. On this point, Peckham differs from many other biblical scholars, who say that the Deuteronomist spoke of "signs and wonders" but did not specify plagues. That's why some say that Joshua 24 is post-Deuteronomistic, even though it contains Deuteronomistic language: it speaks of plagues.
2. On page 346, Peckham states that "The epic covenant is defined as the words that Yahweh spoke on Sinai (Exod 34:27...) and instructed Moses to inscribe on tablets (Exod 34:27-28a), and the sequel refers to it in the same terms (Deut 1:1a...)."
Exodus 34 contains the cultic Decalogue, but Peckham does not appear to attribute all of the cultic Decalogue to the Epic. Rather, from what he says on page 53, I see that Peckham says that the Epic is responsible for Exodus 34's promise that God will dispossess the Canaanites, as well as the rules for the Israelites to (1.) avoid making a covenant with the Canaanites, (2.) worship the LORD only, (3.) give God all of the firstborn (then cattle are mentioned), (4.) rest on the Sabbath, (5.) observe the Feast of Weeks, and (6.) appear before the LORD three times a year. If these are the Epic's terms of the covenant, I can understand Peckham's argument that Isaiah had problems with it because it emphasized worship in the cult, but not ethics.
3. On page 346, Peckham states that, for the Priestly writer, "The sabbath is an eternal covenant (Exod 31:13, 16) and the Priestly alternative to the epic covenant concerning the land." On page 347, Peckham says, "The Priestly writer shared [Isaiah's] disdain for the covenant that ordained political activity and regular worship and presented Yahweh as the God who made the world and rested and who expected people to observe the same rhythm."
My impression from what I have read elsewhere in this book is that, for Peckham, P thought that the Israelites should dwell in the Promised Land. But P's emphasis was more on Israel participating in the order that God established at creation---an order that included resting with God on the Sabbath day. P had a cosmic orientation.