For undergraduate mathematics (semi-opinionated, not exhaustive): Calculus: Apostol and Spivak, take your pick Linear Algebra: Valenza Abstract Alg

For undergraduate mathematics (semi-opinionated, not exhaustive): Calculus: Apostol and Spivak, take your pick Linear Algebra: Valenza Abstract Algebra: Artin Multivariate Calculus: - Vector Calculus, Linear Algebra, and Differential Forms by the Hubbards - Calculus on Manifolds by Spivak For other fields and classes I am not recommending a book because either (1) I don't think there is a clear winner, e.g. as in the case of Real Analysis, or (2) I'm not familiar enough with the books in that field. If you have other recommendations, please add them! These books changed my life in the best of ways.