If that was a piece of historical fiction, we might say it's an implausible plot. But it's not fiction. The biography's subtitle is right - Hannah More lived an extraordinary life.
So, Hannah More's life was extraordinary for a woman of her times. She never married, but the man who was engaged to her for several years but kept putting off the marriage eventually provided her with an annual income that enabled her to devote herself to writing. She really did work with some of the most prominent people of her time. Her writings, for example the Cheap Repository Tracts, sold in millions.
But as Prior says, Hannah More was a conservative reformer, not a revolutionary. I can see why she hasn't been promoted as a 'proto-feminist' like some of her contemporaries. And her writing style was certainly a product of her time. I read a bit of "The Shepherd of Salisbury Plain" (free via Project Gutenberg) and quickly saw why More's writings no longer appeal to modern readers. Such an openly didactic style won't do these days; even when we wish to influence the opinions of others through our stories, we tend to prefer a much more subtle approach.
Thus, Hannah More has been somewhat forgotten in our days. I'm glad that Karen Swallow Prior wrote this biography of her, as I also think Hannah More is worth knowing. I admire the way More used her talents and resources in accordance with her convictions. That was the point she was striving for, really: to do one's best with the God-given talents one has.
Such an interesting life, and a well-written biography. Heartily recommended.