Which type of reference interview does not fall under common categories?

The choice of "reading a book" stands out as it does not align with the traditional categories of reference interviews. Reference interviews are structured conversations that librarians have with patrons to determine their information needs or preferences. Typically, these interviews aim to gather information to assist in specific inquiries or to guide patrons toward suitable resources, which is characteristic of the other options.

The research project interview, readers' advisory interview, and ready-reference interview are all focused on responding to direct user inquiries or providing guidance. The research project interview typically engages with patrons looking for extensive information on a particular topic and involves a deeper inquiry process. The readers' advisory interview assists patrons in finding books they would enjoy based on their reading preferences or interests. The ready-reference interview addresses immediate, short inquiries for factual information.

In contrast, "reading a book" refers to the act of engaging with a text and is more of an activity rather than a method of information assessment or inquiry. Therefore, this choice does not fit into the framework of a reference interview, making it the correct response.