Overall, "Differential Geometry and Its Applications" by John Oprea is an excellent resource that provides a comprehensive introduction to the field of differential geometry and its applications. The PDF version of the book offers several advantages and can be a valuable resource for students and researchers.
The prose is conversational. Oprea writes like a mentor sitting next to you, often pointing out
The "crown jewel" of the book, which links a surface's local geometry (curvature) to its global topology (how many holes it has). Geodesics: