The Book of Genesis does not mention Moses but in the middle 3 books Moses as the main protagonist and prophet is referred to in third person. In Deuteronomy however Moses speaks in first person mainly. Why the switch?
Technically, Deuteronomy is written in third person.
- The first five verses are in third person, ending with "Moses began to expound this law, saying:"
- Moses speaks from chapter 2 through chapter 30, and the main narration begins again with chapter 31 with occasional dialogue
- Moses recites a poem/song in chapter 32:1-43, and then the narration begins again in verse 44
- In chapter 33, Moses blesses each tribe of Israel, and each blessing has a narrative, third person header.
- Chapter 34 is all third person narration
So, to sum up, Deuteronomy is technically written in third person, but it seems like first person because it records Moses' last words.
Deutoronomy or Debarim (The Words) is, what Moses spoke to Israel towards the end of their travels through the wildernis and deserts of Arabia, and it were his final words to them before he died and before they entered the land.
So much of it is direct speech it would have been unnatural, had he spoken of himself in third person after all these (almost 40 years). Moses - I believe - was not in danger anymore that he would transgress by exalting himself the way it had happened when he and Aaron had exalted themselves before Israel at Meriba.