What are "abrogating and abrogated" and why were they brought together and not each one of them in a separate book?

The definitions of the abrogating (Nasikh) and the abrogated (Mansukh) have been previously established, as have the categories of abrogation. Discussing one category without the other is not possible due to their interconnectedness; the abrogating necessitates an abrogated, and in most cases, the abrogated has an abrogating. This very interconnectedness is what has led authors to combine them when discussing them.