Reference books aren’t the same as textbooks. And, typically, there are several levels of textbook. e.g. high school, then freshman physics, undergrad electrod

Reference books aren’t the same as textbooks. And, typically, there are several levels of textbook. e.g. high school, then freshman physics, undergrad electrodynamics, then Jackson, then specialist references. My point is you go over the subject completely several times, at increasing levels of sophistication.