I am wondering what resources-books/websites-you'd recommend for a
beginner. Something that educates me both about "how" to make
sourdough, and also "why" I am making it like I am--both chemically,
and historically.

Also, I am confused about soured dough and yeast. Was soured dough
originally a replacement for yeast, or were both used? Because in my
recipe books I am surprised to see that the recipes call for both
soured dough and yeast. Is this always the case?

Thank you.