The Dutch middle ages receive far less attention until the end of the period as they ought to be given the area's later importance. Part of the reason may lie in its political fragmentation among the seven provinces of later fame (anticlockwise, Holland, Zeeland, Utrecht, Gelderland, Overijssel, Groningen and Friesland, not including the parts of Brabant, Limburg and Flanders added in 1648), of which the most populous by far (Holland) contained barely a quarter of a million inhabitants as the modern era dawned, about 0.3% of Europe's people: other than language, the Dutch region was united only in its allegiance to the Empire over most of the period, each province otherwise largely going its own way except for periods of personal union under territorial lords.

The period’s political history is thus inevitably convoluted: while Frankish rule extended through Brabant into Gelderland as early as the 5th century, the coastal regions remained under native Frisian rulers until Charlemagne’s conquest in the 8th. The county of Holland emerged from a brief Viking overlordship in the 9th century as the Imperial fief of west Frisia, around the time of the creation of the duchy of Hamaland, later Gelderland. Utrecht came by the 11th century under the control of its bishops along with Overijssel (then including today’s province of Drenthe and parts of Groningen), while Zeeland became a county in its own right upon separation from Holland in 1299. In the 15th century Holland, Zeeland and briefly Gelderland passed into the hands of the Burgundian dynasty, and thence under Habsburg overlordship, to be followed in 1528 by Utrecht and its dependencies.

Another reason for the oversight doubtless lies in the earlier importance of the southern part of the Low Countries and its strategic importance adjacent to to the borders of the Holy Roman Empire and France. England too became drawn into regional politics surrounding Flanders, in the 14th century Europe's most urbanised, industrialised and prosperous region alongside parts of northern Italy, processing the wool that was England's principal export until the emergence of a domestic cloth industry. The north by comparison long remained generally something of a geopolitical backwater after its absorption into the Imperial patchwork, despite upheavals from territorial conflict between the count of Holland and the bishop of Utrecht in the 11th century to Holland’s bitterly-contested 14th-century succession crisis in which England’s Edward III still managed to involve himself (naturally claiming the county for himself) despite the distractions of war with France and the Black Death.

The people of both regions preferred generally to rub along with their provincial lords (though the friction was at times very real, erupting at times into open revolt) while laying the basis of future prosperity. The old Frisian regions long retained a tradition of representation, sharing in the wider western European rise of urban political aspirations after the 11th century and the later emergence of parliamentary institutions exemplified in the 15th-century States of Holland and Gelderland and the subsequent states-general of the Burgundian Netherlands, representing the elites of country and town. But the real Dutch story at least from the 11th century is increasingly (like that of Flanders rather earlier) an economic one, foreshadowing later developments in England and in Europe as a whole, in which the north would take advantage of its exceptional position at the meeting-place of the Rhine basin and the North Sea (we still use the Dutch name) with its ready access to the Channel and Baltic trades.

The lead was taken by Holland, where reclamation of new lands from the sea stimulated the emergence of an unusually favoured peasantry whose agricultural innovations were later to be adapted by other successful economies (notably England). By 1300 the province counted among Europe's most urbanised territories, supplementing its lucrative North Sea herring fishery and expanding dairy production with trade as far as the Baltic and exporting fish, cheese, beer, cloth, bricks and tiles across northern Europe in exchange for grain and raw materials for its growing urban population and industries: van Bavel estimates that Holland by 1500 provided 60% of shipping passing through the Danish Sound. Demand for vessels stimulated the rise of the domestic shipbuilding industry which was to underpin later Dutch maritime power, commercial and naval. Outside Holland, the cities of Utrecht, 's-Hertogenbosch, Groningen, Maastricht, Nijmegen and Zwolle shared in the general upswing, drawing on the agrarian resources of their respective provinces and forming part of a wider urban trade network extending across the Low Countries and into northeastern France and northwestern Germany. By 1550 a quarter or more of the now over a million inhabitants of the northern Netherlands lived in towns, a proportion matched only by Flanders, Brabant and Italy and not to be reached by England for two centuries.

The real Dutch "Golden Age" was of course yet to come, starting in the latter half of the 16th century and continuing through the 17th. But the commercial, maritime and financial attainments of the later period built on the foundations of the 11th-15th centuries, and were in turn to provide elements in the wider European take-off spearheaded by England from the 18th.

A good introduction (though concentrating on the later period) remains (for Holland) Bas van Bavel and JL van Zanden, The jump-start of the Holland economy during the late-medieval crisis, Economic History Review 57:3 (2004), and more generally van Bavel, The medieval origins of capitalism in the Netherlands, Low Countries Historical Review, 125:2-3, (2010). Richard Paping offers higher urbanisation levels for legally-defined towns (as opposed to those of a given size): General Dutch population development 1400-1850, University of Groningen 2014. I found no general treatment of the administrative/political history, so I’ve relied on standard reference works and articles relating to individual provinces - more details available on request.

Do you mean today's Kingdom of the Netherlands or the Low Countries including modern Belgium, Luxembourg and some adjacent areas? I imagine it's the former, but it's always worth checking when the same term can mean different things.