For people who want to learn or teach mathematics on the basis of reason …
Santos (who died in 2011) and I were colleagues at Community College of Philadelphia and we often collaborated, tacitly as well explicitly, even if not always easily. But both of us deeply loved mathematics and, if my knowledge of mathematics might have been a bit broader---I went from Fluid Mechanics to Category Theory, his was decidedly much, much deeper. And he wrote an incredible amount in an amazingly short time. And his command of LaTeX was unmatched: his texts are a pleasure just to look at.
Both of us wrote for the students but this is where we fiercely differed: style-wise I am resolutely on Euler's side while his style was very much Edmund Landau–––although he did mellow quite a bit in his revisions.
For instance, on the matter of defining functions, here I am after having introduced relations:
and then, after an example of a function specified by a table,
And here is Santos from scratch:
I think that both the similarities and the points of departure are telling so I need say no more. Both the pdfs and the sources of Santos'texts can be downloaded from this site.