What is "the Book" (al-Kitāb) referring to in this āyah and what did they disagree about?

Imām Ibn al-Jawzī (d. 597/1201) summarized the scholars' opinions in his book "Zād al-Masīr" as follows:

1. It refers to the Torah, and in their disagreement over it, there are three views:

1a. The Jews and Christians differed over it; the Christians claimed it contained the description of Jesus, while the Jews denied that. 1b. They opposed what the Torah described about Muḥammad (ﷺ). 1c. They disagreed with their predecessors in adhering to it.

1. It refers to the Qurʾān, with some of them saying it is poetry, while others claimed it was taught by a human.