That's a good question. In the spirit of making learning easier it would perhaps "start" with what we teach children. However, that might be ultimately misguided because that would just mimic the curriculum and force the whole thing into a box. But then again the math knowledge itself forces a structure on any such textbook because to know A you need to know B, C, D, etc.

This is further complicated by the fact that most subjects in math can be approached from multiple equivalent angles. For example, complex analysis can be taught from the complex derivative perspective or from the power series perspective equally well. Ideally the format of the whole thing will allow the student to naturally learn things from the angle they understand better.