What is the reason for the variation in the spelling of a word in the Holy Quran from one verse to another, such as the word "المَلَؤُ" (al-mala’u) and "المَلَأَ" (al-mala’a), and what are the books that have addressed this aspect?

The script of the Holy Qur’an is tawqifi (divinely ordained and unalterable). It was written during the time of the Messenger of Allah, peace and blessings be upon him, and he approved it. If some words differ in their spelling, it is for a wisdom intended by Allah.