Muslims are obligated to believe in all Inspired Books that God has sent down to serve humanity, as delivered through His Prophets. Every Prophet received Inspiration from God. Some Revelations were later compiled to form the content of larger Books such as the Holy Quran, Gospel, Torah, and the Zabur.
The Arabic word for Inspiration or Revelation is Wahi which has several meanings. Wahi means an idea or something revealed or written, commandment, suggestion, to point out something, or to send a message. Inspiration or Revelation comes to God's Human Prophets either directly or through an intermediary of the Angel Gabriel, who brings the Revelation to them. Every religion that believes in God believes in Revelation.
God's Books to mankind contain Commandments of God, prohibitions, exhortations, stories, parables, reminders, descriptions of Himself and His Attributes, descriptions of the afterlife, Heaven and Hell, the purpose of life, the creation of the Universe, Worship, Piety, morals, manners, the importance of being kind to one's parents, and much more.
These Books seek to guide man through every aspect of life. God's Books act as a guide, an instructional manual regarding how one's life should be lived. God, in a high manifestation of His Mercy, Compassion, and Love, sent Books to teach and guide humanity.
"...Your Lord is the possessor of vast Mercy..." (Qur'an 6:147)
Islam counts as an article of faith the belief in all of God's Books, in their original, pure form.
“Say, [O believers], ‘We have believed in Allah and what has been Revealed to us and what has been Revealed to Abraham and Ishmael and Isaac and Jacob and the Descendants and what was given to Moses and Jesus and what was given to the prophets from their Lord. We make no distinction between any of them, and we are Muslims [in submission] to Him'"(Quran 2:136)
Unfortunately, the modern-day texts of the Torah and Bible have been altered by men and contain mixtures of humanmade ideas and innovations that corrupt their current state. Whereas these Books still contain some remaining traces of truth, they do not stand in their original revealed form. God warns in his final Testament, the Holy Quran:
"So, woe to those who write the ‘scripture’ with their own hands, then say, ‘This is from Allah,’ to exchange it for a small price. Woe to them for what their hands have written and woe to them for what they earn" (Quran 2:79)
When previous Holy Scriptures were altered and corrupted by human hand, God in his Mercy enlisted his last and Revelation, the Holy Qur'an. God has taken it upon Himself to safeguard & protect his final Book to humanity from human-made alterations or any form of corruption.
The Holy Quran is the only scripture in existence today that exists in its original language and words. Not one letter of the Quran has been changed since its Revelation. The Holy Quran is meant to serve and teach all people until the end of time. Everything found in the Quran is truth, with no evidence of contradictions or falseness, and will remain so for eternity.
Muslims believe that the Koran has abrogated all previous Scriptures before it; meaning it cancels the rulings of the earlier scriptures and renders them inapplicable going forward since the preceding scriptures were meant for the earlier nations and not for us. God, in his infinite wisdom, did not feel fit to send down the Holy Quran at the time of previous nations. As the Creator of the World and everything it contains, God knows who is capable of understanding and who is not. Although different revelations came down to various Prophets and nations, the general Message had always remained the same: 'To worship God Alone and to follow his Commandments.'
The Quran mentions the following Books by name:
Torah was sent with the Prophet Moses (Musa in Arabic) to the Children of Israel
Psalms (Zabur in Arabic) was sent with Prophet David (Dawood in Arabic) to the children of Israel
Gospel (Injeel in Arabic) was sent with Prophet Jesus (Isa in Arabic) to the children of Israel
The Scripture Revealed to Prophet Abraham
The Holy Qur'an was sent with the last and final Prophet, Muhammad to our nation, which is the last nation, to all of humanity. No other scripture will be revealed after it.