A Muslim believes that the Qur'an is the word of Allah (God), but not the only word. Allah sent many prophets before Muhammad and He spoke to them as He spoke to him. So a Muslim also believes (in fact he would not be a Muslim if he did not believe) in these earlier books, like the Torah and the Gospel, since the true believers are those who "believe in what has been sent down to thee (Muhammad) and what has been sent down before thee" (Surah al-Baqara 2: 4).
"Say: We believe in Allah, and that which has been sent down on us, and sent down on Abraham and Ishmael, Isaac and Jacob, and the Tribes, and in that which was given to Moses and Jesus, and the prophets, of their Lord; we make no division between any of them." (Surah al-Baqara 2:136)
Allah (God) created men so that they may serve Him. His being a servant of Allah constitutes the essence of man. Man cannot therefore attain to his true humanity and acquire peace of mind unless he realises this aim for which he was created. But how can he do this! Allah, being Merciful and Just, has helped him in many ways. He granted him as we said before an originally good nature that is inclined to know and serve its true Lord. He granted him a mind that possesses a moral sense and the ability to reason. He made the whole universe a natural book full of signs that lead a thinking person to Allah. But to make things more specific, to give him more detailed knowledge of his Lord, and to show him in a more detailed manner how to serve Him, Allah has been sending down verbal messages through His prophets chosen from among men, ever since the creation of man. Hence the description of these messages in the Qur'an as guidance, light, signs, reminders, etc.
All these books advocated basically the same message,
"And we sent no a Mesenger before you (Muhamad) except that we revealed to him, saying,'there is no god, but I so serve Me.'" (Surah Ambiyaa 21:25)
And the religion which they all expounded is Islam i.e. surrender to God,
"The true religion with God is Islam." (Surah Ali Imran 3:19)
Thus Noah (10:72) Abraham (3:67), Jacob and his sons (3:133), the Apostles (5:111), etc. were all Muslims. Islam in this sense is in fact the religion of the universe.
"What! do they desire another religion than Allah's and to Him has surrendered (aslama = became a Muslim) whatsoever is in the heavens and the earth, willingly or unwillingly, and unto Him they shall be returned." (Surah Ali Imran 3:83)
If the religion of all prophets is the same in its essence and basic foundation, not so are the ways of life based upon it. (Surah al-Ma'idah 5:48) One last important point about books is that with the exception of the Qur'an they have not been preserved intact, but have either been completely lost or else suffered distortion and corruption. As to the Qur'an, Allah has decreed that it shall never be subjected to such distortion but shall be preserved by Him. (Surah al-Hijr 15:9) Therefore, Allah revealed the Qur'an, His last Revelation.
It is the final book of guidance from Allah SWT to mankind sent through His final Prophet, Muhammad SAW. Revealed over a period of 23 years, it contains a total of 114 chapters covering a vast range of topics and subjects. It is the complete guide for living one's life in this world and seeking the pleasure of Allah SWT.The teachings of the Qur'an are universal and suitable for all times and places. Indeed, it is the shining beacon that guides humanity to overcome the darkness of ignorance. Allah SWT also informs us that He in His Absolute Power will substitute His revelations when He so wishes. He expressedly puts this forth in the following Qur'anic verse:
"None of Our Revelations do We abrogate nor do We cause to be forgotten, but We substitute with something better or similar. Do you not know that Allah (SWT) has power over all things?" (Surah al-Baqarah 2:106)
The Qur'an is the the Word of God par excellence. It is the eternal miracle that Allah SWT in His All-encompassing Mercy and Grace will preserve forever so that mankind, as long as we are here, will be able to uphold it and abide by its teachings, hence elevating ourselves to the noblest of destinies.