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Having print books converted to accessible format is commonly too time consuming for a semester length research project, so people with print disabilities may prefer ebooks, when the ebooks are accessible enough for their purposes.
You will often retrieve some results in addition to ebooks in OneSearch. For example, OneSearch pulls up some government documents when you limit to full text online and books. One way to identify that you have retrieved a government document instead of an ebook is that when you open the full text, the web address will have .gov in it. Additionally, the title of the result may allow you to tell that the item is a report from Congress or other government body, rather than an ebook. (However, you may find the government documents helpful.)