moshe rabbeinu - Which part of Tanach did Moses give the people in Dt. 31?

I read in the below verses that Moses himself wrote the Torah and gave it to his people with the ark. The book given by Moses seemed to be very important since he propheisized that people would deviate from this book. Hence it would be beneficial to know which part of the Tanach was part of this book was it the entire Tanach? Also was it seems the book given by moses was not that much voluminous as it could be kept beside an ark?

Deutronomy 31

24 And it came to pass, when Moses had made an end of writing the words of this law in a book, until they were finished, 25 that Moses commanded the Levites, that bore the ark of the covenant of the LORD, saying: 26 'Take this book of the law, and put it by the side of the ark of the covenant of the LORD your God, that it may be there for a witness against thee. 27 For I know thy rebellion, and thy stiff neck; behold, while I am yet alive with you this day, ye have been rebellious against the LORD; and how much more after my death? 28 Assemble unto me all the elders of your tribes, and your officers, that I may speak these words in their ears, and call heaven and earth to witness against them. 29 For I know that after my death ye will in any wise deal corruptly, and turn aside from the way which I have commanded you; and evil will befall you in the end of days; because ye will do that which is evil in the sight of the LORD, to provoke Him through the work of your hands.' 30 And Moses spoke in the ears of all the assembly of Israel the words of this song, until they were finished: {P}

Answer

It was the entire of the 5 books (the Torah), including the remaining 3 chapters

"And it was, when Moses finished writing the words of this Torah in a scroll, until their very completion"

then it was placed inside the ark (though some say next to).