Surah 6:91, "They (the Jews, Quraish pagans, idolaters) did not estimate God with an estimation due to Him when they said: "Nothing did God send down to any human being (by revelation)." Say, (O Muhammed): "Who then sent down the Book which Moses brought, a light and a guidance to mankind which you (the Jews) have made into (separate) papersheets, disclosing (some of it) and concealing much. And you (believers in God and His Messenger Muhammed were taught (through the Quran) that which neither you nor your fathers knew." Say: "God (sent it down)." Then leave them to play in their vain discussions."
The Jews deliberately concealed a lot of information which God gave to them. They have much of this secret knowledge in books like the Zohar and the Jewish Kabbalah. It could be other gospels like the Forgotten Gospel of Eden (Adam and Eve) were also known to the Jews but they chose to deliberately leave this out of the Old Testament Canon. The Quran came to bring back some of this missing revelation which was either rejected or concealed.
Jews claim not all revelation from the Books of Moses (the Torah) is contained in the canon. They believe much more information is in the Babylonian Talmud and the Mishnah commonly called The Oral Law. Yet further information is missing from the Canon and can be found in the Zohar and 3 Enoch (Rabbi Ishmael). Yet alone all the exhaustive commentaries on the Law.
Those who claim the Bible is the complete word of God have to explain this Surah. We have to accept there are books outside the Canon which are relevant to our understanding of God. The essential information is however in the Torah and the Gospels with added commentary now contained in the Quran.
There are so many other gospels left out of the Canon which have some relevance that it no longer becomes tenable to believe the Bible is the complete word of God.
Books like 1,2,3 Enoch also contain information which is relevant to our studies. Much of mysticism was deliberately taken out of the church. The Quran is the Final Testament prior to the ministry of the Two Witnesses primarily for a Jewish Christian audience who would understand its contents. An Angel in Revelation 10 is bringing another "biblical scroll" (verse 2) to the Apostle John. It likely this is a reference to the Quran, the Final Testament which would complete the revelation from God.
The Quran deals with many mystical aspects which is lacking in the Abrahamic faith. Some may interpret this as "gnostic" or esoteric in nature. Additional information is also given about the characters found in the Holy Scriptures. We also get a glimpse of hell, the soul, angels, the gardens of paradise, seven heavens, and gates of hell. We learn of the jinn (demons) who listen to Court hearings in the Heavens so they can know the future of people. King Solomon also makes an appearance summoning up demons and putting them to work around the Temple in the Quran, confirming the Testament of Solomon. We learn later he is regarded as righteous and in the gardens of Paradise.
We have to learn to remove our religious bias against Islam before we commence on a journey to uncover the hidden depths of the Quran. Only then will we learn to appreciate its contents.