Great article. One of my favorite resources, Project Gutenberg, offers some ideas on what is "free". I refer to Gutenberg for a lot of my research. Mostly public domain, mostly available for copying, with some caveats. You can use text and also capture images in screen shots. Follow the guidance of Gutenberg in doing this. Rules vary by country. I captured the image here from Lewis Carroll's Alice in Wonderland (copyright 1916). In most cases, I like to attribute material because I think this is courteous and also because readers often like to know sources. Usually, if readers see you've taken care in preparing your material they are likely to place greater credence in what you're offering.