Free textbooks in many subjects, including math: https://open.umn.edu/opentextbooks/

Most have reviews.

It includes pre-algebra, elementary algebra, elementary geometry, intermediate algebra, college algebra, precalculus, calculus, multivariable calculus, linear algebra, discrete mathematics, finite mathematics, elementary statistics, math for liberal arts, differential equations, introduction to proofs, abstract algebra (one or two semesters), number theory, probability, combinatorics, college geometry (Euclidean plane one, the other one is HS), real analysis (one or two semesters), data structures and algorithms, linear regression/programming

I will draw specific attention to:

Math in Society, a Math for Liberal Arts class that would be an excellent 4th credit for a struggling or uninterested student, and do equally well for supplemental material for a bright younger student.

Open Logic Project, rigorous but non-mathematical, aimed at students in the humanities.

Advanced Problems in Mathematics, which looks like a collection of challenging problems that would be a fine supplement for any student eager for more. Intended to be for those taking examinations at Cambridge, so some problems may rely on A level material, which includes calculus.

Algebra and Trigonometry, which is a college algebra/trig textbook that includes significant review in the first two chapters, ideal for someone who's forgotten a lot.

Calculus for the Life Sciences I and II, which look ideal as a first exposure for someone more interested in those areas.

Proofs and Concepts, designed for undergraduates but suitable for mathematically mature high school students as well.

OpenIntro Statistics, a more advanced text that uses some calculus.