You could do worse than go back and make sure your foundation maths is solid. Revise some Discrete Maths books and understand identification, classification, sets, equivalence... Make sure you've a solid ground on concepts like dimensions, functions, differentiation, integration, extrapolation, interpolation, then toughen up your Linear Algebra, optimisation, solving, regression, before getting into approximation and…
This. Though I have no textbook I'd recommend; all of the ones I used were a very hard slog to read, let alone grasp the maths in them.