The most useful tool is the academic citation graph, e.g. via Google Scholar.

Start with a couple keywords. Click through the "cited by n" links on the top few papers. For papers that don’t have PDFs freely available, find DOIs and put them into Sci-hub. Books can often be found at the Internet Archive, Google Books, or libgen. At the start, skim skim skim.

Look at what links forward and backward from the papers you see. Hunt for new keywords to try. Go a few hops all around the graph. It often doesn’t take too long to get a rough lay of the land.