Deuteronomy was originally part of the historical books such as 1 and 2 Kings, but was later reassigned to the Torah.[1 p.173]
Collins[1 p.159] considers the name "Deuteronomy" (from Greek Deuteros nomos, meaning second law) apt. He analyses the book's structure as:
"Apart from the closing chapters, the book has a far more consistent and distinctive style than the other books of the Pentateuch. Even if it was composed in stages, the scribes who contributed to its growth must have come from the same school or tradition. The style is distinctive in the use of direct address, and a highly personal tone." He adds that the structure and style loosely follow that of 7th century BC middle eastern treaties, suggesting a treaty between the Israelites and YHWH. Also this parallel suggests implying composition around the date when the book of the law was "discovered" and read to King Josiah. Was it Exodus or Dueteronomy that they read to him? The comment on 2 Chronicles 34:14 suggests it was Deuteronomy, which is more humane. Perhaps older verses such as Exodus 22:29 were being used to justify human sacrifice; "...underlying all the minute detail is a grand plan, which was to create a society that was fair and honoured God." .
Collins is developing the thinking of W M L de Wette in 1805, who argued that Deuteronomy is the "Book of the Law" that was "discovered" during Josiah's reign in 2 Chronicles 34:14, which was a revised version of Exodus 21–23 taking into account the centralisation of worship.[6 p.765] See also Old Testament regarding authorship.
The structure of many of the laws in this books is "first, there is a crisp statement of the law, then there is an explanation of how the law is to be carried out, and finally there is an expansion of the law." 
Under Josiah control was centralised at Jerusalem, as required by Deuteronomy 12. Ancient shrines such as Bethel were closed, and the poles altars and trees established by the patriarchs were destroyed. Country people could only sacrifice a few times a year, and even the Passover had to be celebrated at Jerusalem (16:2); Deuteronomy 12:15 allows them to eat meat in a family setting, in contravention of Leviticus 17. The levites of the ancient shrines were unemployed; Deuteronomy 18:6–8 provides for them, and Ezekiel 44 promises their conversion to monotheism.
The book has two introductions, one from 1:1–4:43 (which introduces the entire Deuteronomic history) and the other at 4:44 (which introduces this particular book); 12:1 (the start of "the statutes and ordinances") opens the core of the book, and suggests that this might be the part that was read to King Josiah. Deuteronomy 17:14–20 makes rules for kings, such as not having many horses or wives, which Solomon broke. Deuteronomy 17:18 requires the King to have his own copy of the law provided by the priests, which is what the "second law" was meant to be; but instead of providing a duplicate of Exodus and Leviticus, the priests expanded it into the book we have today.
© David Billin 2002–2021