Lecture Note on Curves and Surfaces, 2005, Chuu-Lian Terng
This is a intro book on differential geometry.
Part 1 is the basics of beginning differential geometry. You just need good multi-variable calculus and linear algebra as prerequisite. (and a little ordinary differential equations)
Part 2 covers pseudospherical surfaces, constant mean curvature surfaces, and minimal surfaces.
Original source is Professor Chuu-Lian Terng's home page at https://www.math.uci.edu/~cterng/
If you have a question, put $5 at patreon and message me.