Deuteronomy is the fifth book of the Bible and is part of the Old Testament. If is also the fifth and final book in the Torah, the theological mainstay of much of modern Judaism. It reiterates much of the narrative and rules discussed in Exodus, Leviticus, and Numbers as well as chronicling Moses's death. Joshua is the next book in the Bible and picks up chronologically where Deuteronomy leaves off.
Deuteronomy was written by Moses, except for the very end, which chronicles Moses's death and the events immediately thereafter. The authorship of this portion of the book is unknown.