Is it permissible for a student to take books from the college library without the knowledge of the responsible employee, to keep them for a long period exceeding the specified borrowing period, and is it permissible for him to keep an important book, of which there are many copies in the library?

It is not permissible to take books from the library without permission, nor to keep them longer than the specified period, because that is an unlawful transgression. Public property is like private property in the sanctity of not transgressing against it. This is based on the Prophet's (peace be upon him) saying: "Muslims are bound by their conditions," and his saying: "Indeed, there are men who delve into the property of Allah without right, and for them is the Fire on the Day of Resurrection." Similarly, it is forbidden to deny a borrowed item, due to the hadith (definition) of the woman who used to borrow items and deny them, so the Prophet ordered her hand to be cut off.