Other Revealed Books
Allah has sent Messengers with revealed books to guide mankind to the Truth and establish no plea of ignorance on the Last Day.
Mankind were one community and Allah sent Prophets with glad tidings and warnings, and with them he sent the Scripture in truth to judge between people in matters wherein they differed. (Quran 2:213)
Messengers as bearers of good news as well as of warning in order that Mankind should have no plea against Allah after the (coming) of the Messengers... (Quran 4:165)
Belief is required in all of the revealed books that came from Allah.
...say: "I believe in whatsoever Allah has sent down of the Book (all the Holy Books; the Qur'an, the Books of Old, the Torah, the Injeel or the Scrolls of Abraham)... (Quran 42:15)
And whosoever disbelieves in Allah, His Angel, His Books, His Messengers and the Last Day, then indeed he has strayed far away. (Quran 4:136)
Some Revealed Books have known names such as the Zabur (the Psalms) of David عليه السلام, Torah of Moses عليه السلام, Injeel (Gospel) of Jesus عليه السلام and the Quran of Muhammad صلى الله عليه وسلم and the names of other revealed books are not known.
It is He (Allah) Who has sent down the Book (the Quran) to you with truth, confirming what came before it. And He sent down the Torah and the Injeel. (Quran 3:3)
...and to David We gave the Zabur. (Quran 4:163)
Verily, this is in the former Scriptures, the Scriptures of Abraham and Moses. (Quran 87:18-9)
Without the Revealed Books, mankind would not be able to work out the purpose of their lives.
Allah is the Guardian of those who believe. He brings them out of darkness into light... (Quran 2:257)
The Revealed Books contain knowledge of God's Attributes, the unseen (including the past and the future), His Admonitions, His Promises as well as His Laws to be enacted by mankind.
This is the Book (the Quran), whereof there is no doubt, a guidance to those who are piously good. (Quran 2:2)
God has sent Guidance to all nations that they have deliberately turned away from for various reasons over time.
To each among you, We have prescribed a Law and a Clear Way. (Quran 5:48)
Books have been continually revealed to confirm the Scriptures of the previous Prophets who (being mortal) died and whose messages were deliberately changed.
Then woe to those who write the Book with their own hands and then say, "This is from Allah," to gain from it a little (monetary) price! Woe to them for what their hands have written and woe to them for that they earn thereby. (Quran 2:79)
Each subsequent Revealed Book contained new Laws which partially or totally nullified Laws from the previous Books; this ended with the Quran as the final and decisive Word of God.
And I (Jesus) have come confirming that which was before me of the Torah, and to make lawful to you part of what was forbidden to you... (Quran 3:50)
The Revealed Books were free from corruptions at the time they were revealed by God to their respective Prophets. The Quran remains uncorrupted and preserved by God Himself.
Verily, it is We Who have sent down the Remembrance (i.e. the Quran) and surely We will guard it (from corruption). (Quran 15:9)
Muslims believe everything in the Quran unconditionally, and also whatever matches its teachings from the Books that preceded it but have become corrupted, as for what does not match the Quran, we neither deny it (as it might be true) nor accept it (as it might be false).
And We have sent down to you (O Muhammad) the Book (this Quran) in truth, confirming the scripture that came before it and (being) dominant over it. (Quran 5:48)