Mathematics (Open Access Textbooks)
OpenStax Calculus https://openstax.org/details/books/calculus-volume-1 https://openstax.org/details/books/calculus-volume-2 https://openstax.org/details/books/calculus-volume-3
OpenStax Precalculus https://openstax.org/details/books/precalculus
OpenStax College Algebra https://openstax.org/details/books/college-algebra
Book of Proof (Richard Hammack, free PDF) https://www.people.vcu.edu/~rhammack/BookOfProof/
Linear Algebra (Jim Hefferon, free textbook) https://www.cs.ox.ac.uk/files/12921/book.pdf
Physics (Open Access Textbooks)
OpenStax College Physics https://openstax.org/details/books/college-physics
OpenStax University Physics (calculus-based series)
Volume I (Mechanics) https://openstax.org/details/books/university-physics-volume-1
Volume II (Electricity & Magnetism / Thermodynamics) https://openstax.org/details/books/university-physics-volume-2
Volume III (Optics & Modern Physics) https://openstax.org/details/books/university-physics-volume-3