Deuteronomy 8 doesn't say a whole lot that's new -- it's just more of the endless exhortation to worship YHWH and not to stray, and more recapitulation of events of Exodus and Numbers. The book generally is very turgid and this chapter is pretty much filler. There are a couple of items worth noting, however.
8 “All the commandment which I command you this day you shall be careful to do, that you may live and multiply, and go in and possess the land which the Lord swore to give to your fathers. 2 And you shall remember all the way which the Lord your God has led you these forty years in the wilderness, that he might humble you, testing you to know what was in your heart, whether you would keep his commandments, or not. 3 And he humbled you and let you hunger and fed you with manna, which you did not know, nor did your fathers know; that he might make you know that man does not live by bread alone, but that man lives by everything that proceeds out of the mouth of the Lord. 4
Jesus is said to have quoted this last line in the Gospel of Matthew. By the way, the journey from Egypt to Canaan, on foot, would take only a couple of weeks. God imposed the forty year exile on the people, it's a lot of chutzpah to take credit for getting them through it.
Your clothing did not wear out upon you, and your foot did not swell, these forty years. 5 Know then in your heart that, as a man disciplines his son, the Lord your God disciplines you. 6 So you shall keep the commandments of the Lord your God, by walking in his ways and by fearing him. 7 For the Lord your God is bringing you into a good land, a land of brooks of water, of fountains and springs, flowing forth in valleys and hills, 8 a land of wheat and barley, of vines and fig trees and pomegranates, a land of olive trees and honey, 9 a land in which you will eat bread without scarcity, in which you will lack nothing, a land whose stones are iron, and out of whose hills you can dig copper. 10 And you shall eat and be full, and you shall bless the Lord your God for the good land he has given you.
11 “Take heed lest you forget the Lord your God, by not keeping his commandments and his ordinances and his statutes, which I command you this day: 12 lest, when you have eaten and are full, and have built goodly houses and live in them, 13 and when your herds and flocks multiply, and your silver and gold is multiplied, and all that you have is multiplied, 14 then your heart be lifted up, and you forget the Lord your God, who brought you out of the land of Egypt, out of the house of bondage, 15 who led you through the great and terrible wilderness, with its fiery serpents and scorpions and thirsty ground where there was no water, who brought you water out of the flinty rock, 16 who fed you in the wilderness with manna which your fathers did not know, that he might humble you and test you, to do you good in the end. 17
Again, God inflicted the fiery serpents on the people (Numbers 21) so it's a pretty cheap trick to take credit for saving them. Same with the lack of water, and the conflicting stories about getting water from the rock (Exodus 17 and Numbers 20).
Beware lest you say in your heart, ‘My power and the might of my hand have gotten me this wealth.’ 18 You shall remember the Lord your God, for it is he who gives you power to get wealth; that he may confirm his covenant which he swore to your fathers, as at this day. 19 And if you forget the Lord your God and go after other gods and serve them and worship them, I solemnly warn you this day that you shall surely perish. 20 Like the nations that the Lord makes to perish before you, so shall you perish, because you would not obey the voice of the Lord your God.
How many times do we need to hear this?