Category

Amazon groups products into categories and subcategories. Some categories require approval, others do not.

A category tag or tag that bloggers use to post to their blog based on a specific topic or niche.

A group, often named or numbered, in which elements are assigned based on similarity or certain criteria.
A set of objects together with a transitively closed set of arrows that can be put together such that each object has an identity arrow and the composition of the arrows is associative.

I wouldn't equate this book with the author's first novel.
Just as a monoid consists of a base set with a binary operation that is closed, associative and with an identity for, a category consists of a base digraph with an arrow compound operation that is transitively closed, associative and with an identity for each object. † If a category's compose operation is limited to only one of its objects, it converts that set of arrow objects (all of which will be loops) into a monoid.