Reading Refers to A Habitual and Regular Reading of Books. The Underlined Word Is a/an:

In the sentence provided, the underlined word "Reading" refers to a habitual and regular activity. To identify the grammatical category of this word, we need to analyze its function within the sentence.

1. Clarification of the Term: The word "Reading" typically serves as a noun in this context. It refers to the act or process of examining and understanding written text.

2. Type of Noun: Specifically, in this case, "Reading" is a gerund, which is a verb form that functions as a noun. It describes the action of reading in a general sense rather than a specific instance.

3. Conclusion: Therefore, we can conclude that the underlined word "Reading" is a gerund acting as a noun in the given context.

Final Answer

The underlined word "Reading" is a gerund.