MIT’s “Mathematics for Computer Science” (2018) remains the best free on-ramp to discrete math rigor
MIT’s 2018 edition of “Mathematics for Computer Science” is the rare free resource that actually meets developers where they live: proofs as tools, not ceremony. What’s notable here is how cleanly the text connects core techniques-induction, invariants, recurrences, graphs, modular arithmetic, counting, and probability-to the things engineers care