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The books are generally not available online, unless you mean at an online bookstore, like Amazon. Textbook companies do not make their books available free. If you want to read the same book which those who take the class are reading, you can, however, do that. You are correct in thinking that you will have to look at the course “syllabus,” which is a list of the course requirements, and usually has the textbook name and author name. The syllabus is available online at many universities, so go to the university website and look for it there. Once you have found the syllabus and you know what the textbook for the course is, you will have to buy a copy, either at a regular bookstore or by “mail order.” There are often used copies available at a discount if you hunt around.