Some of the books that you mention seem a bit too hard for a teen, so you have to be careful not to demotivate them by expecting too much of them; instead i suggest a simpler approach before tackling the big ones; * Functions and Graphs by Gelfand et al. - A small but great book to develop intuition. * Who is Fourier? A Mathematical Adventure - A great "manga type" book to build important concepts from first principles * Concepts of Modern Mathematics by Ian Stewart - A nice overview in simple language. * Mathematics: Its Content, Methods and Meaning by Kolmogorov et al. - A broad but concise presentation of a lot of mathematics. * Methods of Mathematics Applied to Calculus, Probability, and Statistics by Richard Hamming - A very good applied maths book. All of Hamming's books are recommended. There are of course plenty more but the above should be good for understanding.
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