Start with high-confidence differential geometry books, then narrow by subgenre or adjacent topics.
Functional Differential Geometry
Gerald Jay Sussman, Jack Wisdom
Partial Differential Equations
Walter A. Strauss
Differential Equations for Dummies
Steven Holzner
Lie Algebras
Nathan Jacobson
Differential Geometry
Erwin Kreyszig
Partial Differential Equations in Mechanics 1
A.P.S. Selvadurai
