The Holy Roman Empire derived its legitimacy from Charlemagne and the Pope. When the Pope crowned Charlemagne Holy Roman Emperor, it was meant to be a succession of the legitimacy of Rome (which was contested by Eastern Rome based in Constantinople). The Holy Roman Empire was a group of feudal principalities and princedoms that initially were vassals of the Holy Roman Emperor. Over time, the Holy Roman Empire became more decentralized, and the emperor of the Holy Roman Empire became elected by the "Electors" of the HRE, who were a series of Princes that voted on the next emperor.

One of the vassals of the HRE was Austria. The Habsburgs, who ruled Austria, managed to gain a lot of territory directly under their control through inheritance and conquest. They eventually became the major power bloc in the HRE, becoming elected emperor nearly every time.

Meanwhile, in the east, a group of German crusaders had conquered an area in what is now northern Poland, as well as the Baltic countries. They were called the Teutonic Order, and they had conquered the area to convert the locals to Christianity. However, they came into conflict with Poland and Lithuania, and were defeated several times. As a result, their territory was reduced. Eventually, several of the leading knights converted to Protestantism. They reorganized the territory as a duchy under the King of Poland, the Duchy of Prussia. This Duchy was then inherited by the Margraves of Brandenburg, who ruled the area around Berlin. Overtime, this area became known as Brandenburg-Prussia, and gradually annexed and inherited nearby territories, becoming a strong member of the HRE.

In the early 19th century, Napoleon invaded the Holy Roman Empire and defeated the Austrians at Austerlitz. The last Austiran HRE emperor abdicated, and Napoleon created a puppet state out of the former German territories of the HRE, effectively ending the HRE. However, Prussia remained independent, eventually defeating Napoleon at Leipzig and Waterloo. Meanwhile, the former emperor of the HRE had already declared Austria to be an empire: thus, he became the ruler of the Austrian Empire, which by now spanned Southern Germany, Northeast Italy, Croatia, Bohemia/Czechoslovakia, and Hungary.

Prussia and Austria became rivals over their influence in the German states after Napoleon's defeat. In a series of clashes over Holstein, a Danish province that was subsumed into Germany, Prussia defeated Austria and united the northern German states into a federation led by Prussia. Then, with the help of the southern German states like Bavaria, they defeated France in the Franco-Prussian war, As a result, Prussia was in a position to unite all of Germany under the German Empire.

Meanwhile, Austria retained its independence from Prussia, but was having domestic issues, as they ruled over several ethnic groups unhappy with being ruled over by foreigners, especially the Hungarians. After a failed rebellion by the Hungarians, in an effort to appease them and the other groups, the emperor of Austria took the crown of Hungary and ruled both Austria and Hungary as Emperor, thus turning the Austrian empire into the Empire of Austria-Hungary.

I recommend Crankshaw's Fall of the House of Habsburg and Steinberg's Bismarck: A Life for an overview of the transformations of Austria into Austria-Hungary and Prussia into the German Empire. Although those books are more biographical. The HRE is such a hilariously complex subject that I can't begin to put a start to it: You can start with Whaley's Germany and the Holy Roman Empire, but it is HUGE.

The Holy Roman Empire was a political institution that provided a loose affiliation between the German principalities, duchies, bishoprics, and kingdoms of central Europe. The HRE (Holy Roman Empire) was ruled over by the Holy Roman Emperor who was crowned by the Pope and viewed as the successor to the Roman Empire.

The German Empire was a state founded in 1871 after a series of political movements in the mid 19th century advocated for a Ethnic German State. After the dissolution of the HRE by Napoleon in 1806 the German states entered a state of political upheval where the German Confederation was founded in 1815. The alliance of all of the individual states of Germany were seen as weak compared to the other powers in Europe and many hoped to found a unified state for the German people. In 1866 the Austro-Prussian war led to a unified Kleindeuschland (or small germany) with the Prussian government at the head of the state rather than Austria-Hungary. The German Empire was a short lived entity that was dissolved after the German defeat in WWI and only lasted from 1871-1918. Despite this the German Empire was tremendously powerful.

Austria-Hungary was a sprawling multi-ethnic Empire that constituted much of Modern Day Austria, Hungary, Croatia, Serbia, Bosnia, Romania, Slovenia, and portions of northern Italy. The empire was ruled by the enormously powerful and long-lasting Habsburg Dynasty out of Vienna. Since the times of the HRE the Habsburgs had ruled in Austria and maintained many of their ancient feudal rights. Additionally the Habsburg's were staunch Catholics who since the times of the reformation allied themselves with the bourbon monarchies of western Europe in their wars against protestants. This long standing tradition of catholic, authoritarian rule made the Austro Hungarian Empire powerful, but their inability to accept new ideas like Ethnic Nationalism terminally weakened their ability to rule. Eventually this recalcitrant policy of decadent Imperialism fomented rebellian in the Balkans and arguably led to WWI. Along with the German Empire the Austro-Hungarian Empire was dissolved in 1918.

Prussia was a powerful state throughout German History extending into the early days of the HRE. Located at the Northeastern Extreme of the HRE Prussia borders Denmark to the North, Poland to the East, Brandenburg to the South, and Mecklenburg to the west. Originating as the Margraviate of Prussia the ruler of the State was provided an exceptional amount of power, resources and rights to rule as he was tasked with protecting the borders of the HRE. With these rights the Margrave of Prussia became a powerful player in German Politics and would later establish themselves as Kings of Prussia. As a kingdom Prussia prospered and during the reformation Prussia became a powerful ally to the German protestant movement. The power of Prussia became evident during the 19th century when the industrial power of the region allowed Prussia to dominate the region as a major power. Most notably Prussia defeated the Kindom of France in the Franco-Prussian war extending their borders through the Alsance along the border of France. Prussia would later become the German Empire after they united the disparate principalities of Germany under its government, but because of the Protestant/Catholic differences between Austria and Prussia there was no way they could unite.

I can't think of any of the good narrative histories right now. I'll think about it for a bit and try to get back to you!

Are there any documentaries on the history of Germany/Prussia/HRE?