Why did Allah send more than one book to mankind.Explain your Answer. Please consider adding references from the quran and sunnah to your answer.
Allah ﷻ informed us in the Qur'an the reason for having multiple books: legislation. The religion of all books and prophets is Islam. The creed in all revealed books is monotheism (Allah is the only deity to be worshipped). However, legislation was contextual to adapt to the conditions at the time or used as a form of punishment or test.
وَأَنزَلْنَا إِلَيْكَ الْكِتَابَ بِالْحَقِّ مُصَدِّقًا لِّمَا بَيْنَ يَدَيْهِ مِنَ الْكِتَابِ وَمُهَيْمِنًا عَلَيْهِ ۖ فَاحْكُم بَيْنَهُم بِمَا أَنزَلَ اللَّهُ ۖ وَلَا تَتَّبِعْ أَهْوَاءَهُمْ عَمَّا جَاءَكَ مِنَ الْحَقِّ ۚ لِكُلٍّ جَعَلْنَا مِنكُمْ شِرْعَةً وَمِنْهَاجًا ۚ وَلَوْ شَاءَ اللَّهُ لَجَعَلَكُمْ أُمَّةً وَاحِدَةً وَلَٰكِن لِّيَبْلُوَكُمْ فِي مَا آتَاكُمْ ۖ فَاسْتَبِقُوا الْخَيْرَاتِ ۚ إِلَى اللَّهِ مَرْجِعُكُمْ جَمِيعًا فَيُنَبِّئُكُم بِمَا كُنتُمْ فِيهِ تَخْتَلِفُونَ
And We have sent down to you the Book with the Truth, sincerely verifying whatever of the Book that was before it, (Literally: between its two hands) and Supremely Hegemonic over it. So judge between them according to what Allah has sent down, and do not ever follow their prejudices away from the Truth that has come to you. To every one of you We have made a legislation and a program. And if Allah had so decided, He would indeed have made you one nation; but (He did not) that He may try you in what He has brought you. So race with each other in the charitable (deeds); to Allah will be your return, altogether; so He will fully inform you of that wherein you used to differ.
The contents of the Qur'an can be classified into three categories: creed (detailing what Muslims should believe in, and how Muslims should practice their religion), legislation (mainly focused on practice and application of the religion in different aspects of life, commonly referred to as jurisprudence, or fiqh), and narratives (mainly stories of previous prophets, e.g., Yūsuf, Yūnus, Lūt, etc.; or previous people, e.g. the companions of the trench, the two-horned king, the owners of the garden, etc.). According to the Judeo-Christian teachings, the Tanakh is categorized into Torah (mainly dealing with legislation and creed, namely the first five books of the Old Testament), Nevi'im (Nabi-īm, mainly dealing with the stories of the prophets, e.g., Zechariah, Isaiah, Jonah, Ezekiel, etc.), and Ketuvim (ketab-īm, mainly poetic books, narratives, psalms, proverbs, etc.).
Accordingly, as new legislation were needed, books were revealed. In light of the verse above, those books showed people the truth (i.e., Islam) and their creed (i.e., monotheism). In addition, those books were also the standard by which the ruler should judge people on earth (legislation that changed with time), and by which people are judged after resurrection:
وَتَرَىٰ كُلَّ أُمَّةٍ جَاثِيَةً ۚ كُلُّ أُمَّةٍ تُدْعَىٰ إِلَىٰ كِتَابِهَا الْيَوْمَ تُجْزَوْنَ مَا كُنتُمْ تَعْمَلُونَ
And you will see every nation kneeling. Every nation will be called to its Book. (It will be said), "Today you will be recompensed for whatever you were doing.
The legislation on Earth changed with time ("to every one of you We have made a legislation and a program") as revealed by Allah at the time. For example, during the time of Adam, his sons were permitted to marry their sisters (there was no one else at the time and no other options). Later, this permission was abrogated (see Leviticus 18:10) as sexual relations with one's sister became prohibited. Also, prior to the revelation of the Torah, all food was permitted to the sons of Israel (see Qur'an 3:93) then certain foods became unlawful (see Qur'an 6:146). Some of these prohibitions were due to the transgression of the sons of Israel (see Qur'an 4:160 and Qur'an 6:146).
When the full Qur'an was revealed, this completed all of Allah's message to us and there is no need for further revelations:
الْيَوْمَ أَكْمَلْتُ لَكُمْ دِينَكُمْ وَأَتْمَمْتُ عَلَيْكُمْ نِعْمَتِي وَرَضِيتُ لَكُمُ الْإِسْلَامَ دِينًا
Today I have completed your religion for you, and I have perfected My favor on you, and I am satisfied with Islam as a religion for you.
Since that point onward, the focus turned to spreading the message of Islam. As the Prophet ﷺ said, Islam will reach all of the globe:
عن تميم الداري رضي الله عنه قال: سمعت رسول الله صلى الله عليه وسلم يقول: ليبلغن هذا الأمر ما بلغ الليل والنهار ولا يترك الله بيت مدر ولا وبر إلا أدخله الله هذا الدين بعز عزيز أو بذل ذليل عزاً يعز الله به الإسلام وذلاً يذل الله به الكفر
— NOTE: My own translation, so treat with care:
Narrated by Tamīm al-Dārī that he heard the Prophet ﷺ say: "This matter [Islam] will reach as far as the night and day can reach. Allah will not leave a house, be it built of mud or hair, that He will not bring this religion into. glorifyin those who embrace it and disgracing those who dibelieve in it.
— Musnad Ahmad, Hadith 16998
There is another hadith in Sahih Muslim, Hadith 5148 that also says that Islam will govern the Earth from east to west.
NOTE. The religion and creed did not change over time:
إِنَّ الدِّينَ عِندَ اللَّهِ الْإِسْلَامُ ۗ وَمَا اخْتَلَفَ الَّذِينَ أُوتُوا الْكِتَابَ إِلَّا مِن بَعْدِ مَا جَاءَهُمُ الْعِلْمُ بَغْيًا بَيْنَهُمْ ۗ وَمَن يَكْفُرْ بِآيَاتِ اللَّهِ فَإِنَّ اللَّهَ سَرِيعُ الْحِسَابِ
Surely the religion in the Providence of Allah is Islam. And in no way did the ones to whom the Book was brought differ (among themselves) except even after the knowledge came to them, being inequitable among themselves. And whoever disbelieves in the signs of Allah, then surely Allah is swift at the reckoning.
There is only one religion and one creed: Islam. All prophets conveyed the same creed: there is no god but Allah. All prophets likewise preached to their nation that they should become Muslims and die as Muslims (see Ibrahim, Is-hāq and Ismā'īl, Ya'qūb, and his sons, Mūsa, 'Īsa, and so on).
- Books were altered and needed to be corrected. If those books were here today, other than their commandments, its teachings would still be valid, its arguments about judgement day, Tawhid, enjoining good and forbidding wrong would all be valid.
- While the definition of right and wrong didn't change and all needed to be books of guidance, the way you would communicate with people may have changed. At the time of Prophet Jesus medicine was important and at the time of Prophet Moses magic was interesting. At the time of Prophet Muhammad poetry and rhetoric construct was interesting.
The difference between Quran and other books is 1. Its words are God's exact words. The other books are different. Therefore Quran's eloquence and guidance is on a higher level.