I learned set theory in high school from a book by allendoerfer and oakley (principles of mathematics), and books by erich kamke and paul halmos (naive set theory).
Since then I have enjoyed the book of Felix Hausdorff, and the original work by Georg Cantor (contributions to the theory of transfinite numbers.).
a nice work on fourier series is the little benjamin (not an e) book by robert t seeley.
i only know the linear algebra ebooks from having to teach the course and needing a reference for my students.
a suitable one is hard to find. i.e. most decent books assume students know how to read and do proofs, but most students today do not.
the ones i listed run the gamut from mickey mouse (matthews) to excellent (sharipov), and i let the reader choose his own poison.
