A textbook is not merely a book with lots of facts. Textbooks are teaching tools to guide a course. For a given course, the instructor will choose a text, and thus require them for their class. Typically, once the course is over, students either keep them (e.g., for reference) or sell them. They are not usually read merely “for fun,” for they are not “light reading” and they are indeed designed to be read in order according to the professor’s sequence of topics (which seldom, if ever, covers the entire text.)
Until recently, I have (usually) had the quirky “pleasure” of reading them, but without an instructor. I could typically understand them as written, but for anyone to get the full meaning of the text, you’ll need a professor. Typically they are sold or discarded after finishing a course. And even if you buy a used textbook recently, you’ll likely get the same buyer’s remorse I have gotten. On Amazon, both students and non-students have made both positive and negative confessions about textbooks, but the students have the advantage of the instruction. To be modest, I usually keep silent on Amazon reviews nowadays.
According to a recently discarded textbook on Animal Physiology, it was stated that if information is all around is, why is the text and course needed? The direct answer is organization, in order to make sense of the information of a given course that also contains relevant bridges to other fields (e.g., physics and chemistry principles in a biological text).
So, what must the instructor do? S/he will further explain the content in the text (since, again, the text alone may be tough and tedious), focuses on the most relevant topics, and guides you through the course. You will learn much more efficiently with a professor than without. And prerequisites, when applicable, form a foundation for other courses after then.
So no matter how tough you try to read them, textbooks aren’t novels and shouldn’t be treated as such!