I read "the Polish Way" by Adam Zamowski as my first. Tenth century to modern times. One of the best books I have about the commonwealth is actually an art history book. Something something winged hussars. Art of hussars are on the cover. It explains culture of the period and gives a good insight. Trying to think of name and title.
Just wanted to clarify - are you asking why this history of Poland is not taught in US history courses? In the courses of Western European countries? Somewhere else?
From my perspective taking classes in the US, world history teachers were usually struggling to get through everything over the course of a semester. In terms of medieval history, they typically focus on Charlemagne, William the Conqueror, then go on to focus on England, France, the Iberian countries, and the HRE if you're lucky. Part of this has to do with the fact that these Western European countries would go on to have such a significant impact on the world stage, particularly in the Americas, so their history seems to have become much more ingrained in at least American society than that of Eastern Europe. As a result, teachers seem to prioritize that side of history.
Even in my AP European History class, Poland was rarely mentioned - we mostly talked about the partioning of Poland in the 18th century and of course WW2. Poland was certainly a strong regional power at various points in its history, but if any teacher spent time discussing Eastern Europe in my courses, it typically related to the Byzantines, Austria, Prussia or Russia.