I had a quick look through Apostol Calculus Vol 1, I found it online. Of course that is a very good book, a pure mathamatical approach to the development of the calculus, with proofs. Whereas the Thompson book was intended to be a school textbook, "for 5th form boys", as the author puts it. Which tells you something about the maths that was taught to 16 year olds in 1920's grammar schools, compared to today. Whether what the Thompson book teaches is truly "Mathamatics", or merely a series of calculating techniques, is debatable. But sometimes you need familiarity with techniques before introducing more advanced ideas; learning proceeds in stages, not all at once, at least, in my experience.
I think Maths is the science of the abstract, where 'science' means a systematic, organised body of proven knowledge, developed by logical argument. The 'things' that maths deals with are all pure abstractions. I can have '1' of any concrete object, but I can never have "a 1", which is a pure abstraction; by pure I mean an abstraction that can never be instantiated. Hence maths is the science of pure abstractions. What has always struck me as remarkable is that there appears to be a series of pure abstractions, like the pythagoras theorem and many others, that by logical argument provably 'exist' within the abstract realm; they are eternal and never changing, have a characteristic that we perceive as beauty (which is interesting of itself), and are not subject to the entropy that we have observed in the natural world. Do the pure abstractions of mathamatics, exist independently of consciousness? I don't know the answer; but, perhaps not.
Maths is not a natural science. The natural sciences (sometimes) apply some parts of mathamatics to the natural world, to make arguments about the natural world; for example, newton's laws of motion. You could, for example, develop the entire science of mathamatics, without ever having developed any of the natural sciences, if you so wished.
That is my limited understanding, anyway, speaking as a non-mathamatician.