Math
Mathematics is built as a ladder: each book assumes only what came before it. Start wherever matches what you already know — every book states its prerequisites on its cover page, as guidance rather than gatekeeping.
Available books
The starting rung — arithmetic foundations and the first ideas of algebra. Assumes no prior coursework.
A first full course in algebra — real numbers, linear equations and graphs, polynomials, factoring, radicals, and quadratics. Follows on from Prealgebra.
A second course in algebra — functions, systems, polynomials, radicals, quadratics, exponential and logarithmic functions, conics, and sequences. Follows on from Elementary Algebra.