In Exodus, the focus was on coming out of Egypt.  Numbers recorded the wandering in the desert between Egypt and the Promised Land.  Deuteronomy was preparation for entering the Promised Land.  In Joshua, we read of the fulfillment of God’s long-standing promise to give the land to the descendants of Abraham, Isaac, and Jacob.  The corresponding call of the book is for God’s people to be faithful to keep his commandments.  Although the book ends on a pessimistic note about their ability to obey, it also points forward to another Joshua who would finish the work.