Eleh ha-devarim, or “these are the words”, tells us that these words are a written copy of the spoken Word transmitted directly from God to the Israelites through Moses.
The English title, Deuteronomy, is from the Greek and is a misnomer thanks to mistranslation of mishneh ha-torah ha-zoth, or “a copy of this law”. It is a written copy of the law orated by God, but it is NOT a second law!
Furthermore, Deuteronomy 6:4 (affectionately known as the Shema) has become the definitive statement of Jewish identity:
Hear, O Israel: the LORD our God, the LORD is one.
Sh’ma Yisra’eil Adonai Eloheinu Adonai echad.
An overview of the Book of Deuteronomy can be found here.