In 1852 a band of farmers from Indiana begin a trip in a wagon train to California. Among them is Alice Muller, a reluctant traveller forced to leave home by her husband's dreams of a better life. It is she who chronicles the journey's hardships, in the course of which she becomes the leader.
Severity reflects intensity, not value — “central theme” means a warning is a core part of the book, not that the book is bad.