Comment by mna_

Some more free ones:

Abstract Algebra by Judson - https://judsonbooks.org/abstract-algebra-theory-and-applicat...

Linear Algebra by Hefferon - https://hefferon.net/linearalgebra/

Real Analysis by Lebl - https://www.jirka.org/ra/

Differential Equations by Lebl - https://www.jirka.org/diffyqs/

These should be enough to give you a first and second year undergrad level education in mathematics (minus stats&probability).