First of all Polish bread isn't any specific kind of bread, it rather refers to the quality, taste and methods of bread making in Poland (more traditional, of course, the bakeries they have been using machines for a long time, but it's all those chemical raising agents that make "western bread" so bad, it only looks nice)
And the most selled today is probably the standard white bread (wheat flour) on yeast, but if it's made in a natural way, it still tastes a lot better than wheat bread abroad. Other types are wheat-rye, pure rye isn't very popular.
But it's the best IMHO, integral rye bread without chemical additives, it's heavy and it has a little acid taste, indeed, because that's how you make it, you don't use yeast or anything similar, but "zakwas (zaczyn) chlebowy"
here's a Polish wiki site (very short) for zakwas:
Sourdough (leaven) - a small amount of dough rye, left from the previous baking or prepared sourdough from rye flour and containing propagated lactic acid bacteria and wild yeast used to make the dough.
With microelements (including iron, zinc, copper, cobalt, manganese), the phytic acid forms insoluble salts (phytates), thereby blocking their absorption.
and the English one, which mentions yeast, so it's not exactly how it's supposed to be, but I don't bake at home, I'm lucky to have a few good bakeries in my city
most online recipes are with yeast, but I found some that may be OK:
and if you want some more traditional recipes, here's one:
(it's from 1889
vademecum "Perfect housewife")