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