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For freely available online textbooks on a variety of courses, LibreTexts is a great resource, especially to begin exploring a subject.
They're mostly accessible to non-specialists and have many advanced features: https://libretexts.org/advanced.html
Examples:
Astronomy and Cosmology: https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology
Physics: https://phys.libretexts.org/Bookshelves
Math: https://math.libretexts.org/Bookshelves
Biology: https://bio.libretexts.org/Bookshelves
Humanities: https://human.libretexts.org/Bookshelves