Comments added to further describe aspects of the book. If there is a bibliography and how many pages. Can be crucial information. An item with a bibliography tells you where the author got information and can point you to the same sources. Library of Congress subject headings are used to describe items.
Subject headings are consistent, descriptive key terms assigned across a body of information.
With a controlled, limited, systematic, stylized use of words one can create a thesaurus, a controlled vocabulary for description and specific subject headings. Despite this an element of human bias is present in each record. Subject heading assignment is not an exact science. Currently there is a 4-volume set of these headings and every library that uses these subject headings will own a set of these volumes. All proper names are subject headings.