Book fonts are roman style fonts that are designed for use in masses of type with fairly long lines of text, as in books. There is not a separate race of book fonts but old style, transitional, and modern Romans exist with proportions suitable for use as a "book" font.
Typically a book font has a tall x-height in relation to the ascender and many have a short descender length to clear the space between lines for printing the lines closer together without collision between the descenders on one line and the ascenders on the next.
This font is a Modern Roman. Century schoolbook has been used to print 99% of the elementary school books produced in the US during the twentieth century. Notice the characteristically short descender on the p.
A classic font based on Garamond, Bookman Old Style is used for a variety of book publishing applications. It has the tall x-height and the wider letter spacing required when printing large blocks of readable text in small sizes.
