Augustus De Morgan was an eminent English mathematician and logician of the mid-19th century. The Project Gutenberg catalog currently contains six of his books. Although De Morgan writes in a formal (and highly gendered) style by standards of 2025, his aims in the two books showcased here are admirably pedagogical.
On the Study and Difficulties of Mathematics is a 2013 digitization of a 1910 new edition of an 1831 treatise, written for teachers and students, on algebraic and geometric reasoning, with emphasis on philosophy and meaning rather than on rote manipulation.
Elementary Illustrations of the Differential and Integral Calculus is a 2012 digitization of an 1899 new edition of an 1832 exposition. Considerably less famous today than Calculus Made Easy by Sylvanus P. Thompson, the book nonetheless covers similar ground with comparable aims of making calculus more widely accessible to students and the reading public.