The reading passages on the TOEFL can come from almost any subject that first-year university students could take a course in. Here’s a list of some of the possible subjects:
Professional and Pre-Professional Disciplines
Literature (the study of literature, not literature itself—there won’t be any Shakespeare on the test)
Remember that although the topics are varied, the passages are chosen to be understood by somebody who has no experience with the subject. Most of the passages will deal with broad concepts that are fundamental to understanding the subject. You won’t need to learn or demonstrate any skills on the test—that is, if you had a passage from a physics text book, you wouldn’t be expected to learn the material and solve a physics problem. Instead, you would be asked about the big ideas, and, often, details or examples that help to explain those big ideas.
You will, however, probably still find it easier to answer questions about a topic you’re already familiar with. You will have to reread less, which will save you time, and you may be able to fill in information that you missed with your prior knowledge. So while you don’t need to run outside right now and buy a book about every topic that could be used on the TOEFL, do make an effort to vary your reading material. That way, you’ll have at least some familiarity with some of the reading material on test day, and your study sessions will be more interesting, too!