What is the ruling on leaving books and Qur'ans, which I took from an Islamic society, on the wall of the society's president's house after its dissolution, and what must I do to make sincere repentance and absolve myself from this wrongdoing?

The books and pamphlets must be dealt with according to the purpose for which they were intended by the donors and officials of the dissolved association. You erred by placing the books—including the Holy Qur’an—on the house wall, which could lead to them falling or being disrespected. Therefore, you must ensure they reach the head of the association and inform him that they are from its books, so he can deal with them as designated. Alternatively, you can take them from him yourself and dispose of them in the same manner, and you are not obligated to inform him about the matter of taking them without permission.