The Book meant in this verse is the Holy Qur’an. Not a single Book was revealed except that it confirmed the previous Books, as it is from Allah U, from the One God:

﴿شَرَعَ لَكُمْ مِنْ الدِّينِ مَا وَصَّى بِهِ نُوحاً وَالَّذِي أَوْحَيْنَا إِلَيْكَ وَمَا وَصَّيْنَا بِهِ إِبْرَاهِيمَ وَمُوسَى وَعِيسَى أَنْ أَقِيمُوا الدِّينَ وَلا تَتَفَرَّقُوا فِيهِ﴾

“He has ordained for you of religion what He enjoined upon Nuh and that which We have revealed to you, [O Muhammad], and what We enjoined upon Ibrahim and Musa and ‘Isa – to establish the religion and not be divided therein.” [Surah Asy-Syuraa:13].

The Din or religion is therefore from Allah.

The Jews in Medina used to await the coming of a Messenger. When disputes happened between them and the tribes of Aws and Khazraj in Medina, they would say to them, “When the promised Messenger comes, we will be with him and kill all of you the way the people of ‘Aad and Iram were killed!” However:

﴾فَلَمَّا جَاءَهُمْ مَا عَرَفُوا كَفَرُوا بِهِ﴿ “but [then] when there came to them that which they recognized, they disbelieved in it”.