Her two most well known books, A Serious Proposal to the Ladies, for the Advancement of Their True and Greatest Interest (1694) and A Serious Proposal, Part II (1697), outline Astell's plan to establish a new type of institution for women to assist in providing women with both religious and secular education. Astell suggests extending women's career options beyond mother and nun. Astell wanted all women to have the same opportunity as men to spend eternity in heaven with God, and she believed that for this they needed to be educated and to understand their experiences. The 'nunnery' style education she proposed would enable women to live in a protected environment, without the influences of the external patriarchal society.
Her proposal was never adopted because critics said it seemed "too Catholic" for the English. Later her ideas about women were satirized in the Tatler by the writer Jonathan Swift. While the writer Daniel Defoe admired the first part of Astell's proposal, he believed that her recommendations were "impracticable." However, Patricia Springborg notes that Defoe's own recommendation for an academy for women as detailed in his Essay Upon Projects did not significantly differ from Astell's original proposal. Despite this, she was still an intellectual force in London's educated classes.
A few years later, Astell published the second part of A Serious Proposal, detailing her own vision of women's education for courtly ladies. She broke away from the contemporary rhetorical style of the period where orators spoke before an audience for learning, and instead offered a conversational style of teaching "neighbors" the proper way of behavior. She referred only to the Port-Royal Logic as a source of contemporary influence, though still relied upon classical rhetorical theories as she presented her own original ideas. In her presentation, she offered that rhetoric, as an art, does not require a male education to be master, and listed the means of which a woman could acquire the necessary skills from natural logic, which established Astell as a capable female rhetorician.
In the early 1690s Astell entered into correspondence with John Norris of Bemerton, after reading Norris's Practical Discourses, upon several Divine subjects. The letters illuminate Astell's thoughts on God and theology. Norris thought the letters worthy of publication and had them published with Astell's consent as Letters Concerning the Love of God (1695). Her name did not appear in the book, but her identity was soon discovered and her rhetorical style was much lauded by contemporaries.