External Resources
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Free Books
Nearing's Math Methods
Level: Lower-Division Undergraduate
Concised overview of the mathematical tools often used in physics. Mostly focused around calculus topics such as differential equations and integral transformations.
Fitzpatrick's Teachings
Level: Lower-Division Undergraduate - Graduate
Variety of physics teachings including classical mechanics, electricity and magnetism, and quantum mechanics taught at various difficulties.
Recommended Readings
Linear Algebra Done Right - Axler
Level: Lower-Division Undergraduate
Topics in Linear algebra, constructed from the ground up using the foundations of linear algebra. Assumes a basic understanding of set theory and associated notations.
Linear Algebra and Its Applications - Strang
Level: Highschool
Topics in Linear algebra, constructed from a top-down approach using examples and applications, eventually building to the foundations. This is an easier alternative to Linear Algebra Done Right. There are also supplemental lectures hosted by MIT.
Other
Library Genesis
If a book proves to be financially prohibitive and unavailable at your local libraries, there are "shadow libraries" containing a large number of downloadable PDFs and scans. Library Genesis is one of the largest databases of scholarly books and documents. Note that downloading books from such websites is in violation of U.S and international copyright law.
Sci-Hub
If access to research papers proves to be financially prohibitive and unavailable at your local libraries...