The Open Syllabus Explorer is a great way to find out what books are canonical in any given field you’d like to learn about.
Better World Books often has previous editions of textbooks available for just a few bucks. (Note: textbook versioning is largely a scam – publishers release new editions largely to ruin the secondhand market, and quite often there’s little or no new content. You can usually find out quite easily whether the previous edition will suit your needs by checking reviews etc.)
Organising your research
Zotero is a free tool to “collect, organize, cite, and share research,” equivalent to EndNote (used in many universities)
Access to research sources
Open Access Button web browser extension helps you find or request free versions of research papers.
Twitter: use the #icanhazpdf hashtag (along with relevant tags for your field, eg. #histsex #queerhist)