In the preface to Incidents in the Life of a Slave Girl, Jacobs uses a rhetorical device common to a number of slave narratives. In several places, she emphasizes her lack of ability when it comes to writing. This could be seen as corresponding closely to the rhetorical mode of persuasion known as pathos, an appeal to the audience's emotions. In emphasizing how difficult she finds it to write, Jacobs is attempting to win the reader's sympathy, making it more likely that they will respond positively to the book's underlying message.
Pathos is the most appropriate mode of persuasion here as Jacobs lacks the confidence to appeal on a rational basis to those much better educated than herself. Later on in the story, Jacobs will use ethos as a way of kindling the reader's ethical response to the catalog of cruelty and exploitation she sets out in such vivid detail. But for now, in the preface, pathos is very much the order of the day.
The main reason for this is that Jacobs wants her white readers to get the anti-slavery message out and spread it as far and wide as possible. The book that she's written is therefore just the beginning of a collective endeavor. Harriet Jacobs has played her part; now it's up to white abolitionists to get out there and tell people living in free states of the true, horrifying nature of slavery and why it needs to be abolished as a matter of urgency.