As elsewhere in the thread, I'd advocate for "basic higher mathematics" or "introductory higher mathematics" (which would make clear that it's for people actively studying math as a subject and not as a standard part of primary or secondary education, or a prerequisite in an engineering major or something).

The author says that this is largely aimed at high school students who are doing self-study, which is a realistic audience but not a context where a lot of people would naturally apply the word "basic". But this material is basic for mathematicians, I guess (although even a lot of mathematicians may not have quite as broad a knowledge of mathematics as the author does!).