Why not just one big book?
The scriptures which comprise the Old Testament were written between 2000BC AND 400BC
They were written and maintained by the people of Israel.
The scriptures which comprise the New Testament were written in the 1st century AD and maintained by Christians.
Although the people of Israel refuse to accept Jesus Christ as their Messiah, he nonetheless clearly fulfilled prophecies given in the Old Testament. Both testaments are necessary to give a complete understanding of the purpose of God and his offer of salvation to mortal men and women.
Jesus told his disciples:
Luke 24:25… O fools, and slow of heart to believe all that the prophets have spoken: 26 Ought not Christ to have suffered these things, and to enter into his glory? 27 And beginning at Moses and all the prophets, he expounded unto them in all the scriptures the things concerning himself.
Luke 24:44… These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.
He told the Jews:
Luke 16:31… If they hear not Moses and the prophets, neither will they be persuaded, though one rose from the dead.
The apostle Paul told Felix:
Acts 24:14 But this I confess unto thee, that after the way which they call heresy, so worship I the God of my fathers, believing all things which are written in the law and in the prophets:
The writer to the Hebrews affirms the continuity between the Testaments:
Hebrews 1:1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, 2 Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds; 3 Who being the brightness of his glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged our sins, sat down on the right hand of the Majesty on high;
Both sets of scriptures were written by inspiration of God.
2 Timothy 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: 17 That the man of God may be perfect, throughly furnished unto all good works.
Both Testaments are the word of God.
1 Thessalonians 2:13 For this cause also thank we God without ceasing, because, when ye received the word of God which ye heard of us, ye received it not as the word of men, but as it is in truth, the word of God, which effectually worketh also in you that believe.
I hope you find this helpful.