This function returns a simplified data frame of only the unnested section and text columns of a data frame returned by epub, with the text included only up to the first n characters.
This is useful for previewing the opening text of each e-book section to inspect for possible useful regular expression patterns to use for text-based section identification.
For example, an e-book may not have meaningful section IDs that distinguish one type of book section from another, such as chapters from non-chapter sections,
but the text itself may contain this information at or near the start of a section.