The author's primary purpose in the passage is to

While the passage may mention some self-help books, its primary aim is not to endorse or recommend specific titles. Instead, it analyzes the broader trends and historical context of the self-help movement, which does not imply a recommendation of individual works.

Although the author may convey some positive sentiments about the self-help movement, the passage does not primarily focus on expressing appreciation. The emphasis is instead placed on tracing the historical development rather than providing subjective opinions about the movement itself.

While the passage may include some comparisons between different eras of self-help literature, its central purpose is not to conduct a direct comparison. Rather, it aims to provide a historical overview of the movement, which encompasses more than just comparing specific publications from different times.