What is the ruling on writing notes and explanations in books of Fiqh or religious books, while placing one's hand on pages containing Hadith or Quran, and being exposed to sweat that may come upon the verses?

It is permissible to write explanations, comments, benefits, and notes on the margins of religious books, and whatever a student of knowledge needs that is related to them. There is no harm in placing one's hand on the pages when writing or when needed, and this is not a degradation of the books, provided one is careful about sweat and the like. It has been previously explained that it is permissible to write notes on the margins of the Mushaf (Qur'an) that are related to it, such as an exegesis, a legal ruling, or a Tajweed rule.