Google Scholar (http://scholar.google.co.uk) provides access to academic journal articles freely available on the web.
These have been written by experts in the field and contain useful critique/discussion.
Type in your key words and click Search.
The search results that have links on the right-hand side are the ones you can use for free.
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Google Books (https://books.google.co.uk/) provides selected pages from some text-books and full-text of public domain (copyright free) books.
The introduction is often available full-text and can make good background reading.