As a student, you may have access to great resources through your school’s library (definitely check there first, especially for papers). If not, a lot of textbooks, papers, and resources can be found online for free. Here are some websites to check.*
For peer-reviewed/academic papers, also consider contacting the lead author if you’re unable to access a paper you’re interested in.
*Note that downloading from the web does sometimes carry risks. Don’t click on any pop-ups or links with your PDF download should they appear.
Directory of Open Access Books
Project Gutenberg (mostly not textbooks)
Standard eBooks (mostly not textbooks)
Mobilism ebook sharing forum
Paper Panda - Chrome ext. finds pdfs
Directory of Open Access Journals
Research Rabbit can sometimes find free pdfs
Google Scholar also sometimes has links to pdfs