Sarah's Reading Practices
She was likely reading the London 1737 edition of Rollin's The Method of Teaching and Studying the The Belles Lettres, which can be surbmised from her quotation here.
The 1737 London edition also happened to be held by the Library Company of Philadelphia , as evident by the 1835 catalog. The Library Company was began as a subscription library in 1742. As noted by the library company (see finding aid here), Robert Waln — Sarah's father's cousin and buisness partner — was actively involved in the library in its early years, suggesting that she might have had access to the library resources.