One problem with producing a holistic book on the HRE was that the lifespan of it transects various historical epochs. This makes it difficult to achieve a synthesis as a historian might be an early modern specialist, but less familiar with medieval history. Heart of Europe: A History of the Holy Roman Empire by Peter H. Wilson. But such an extensive history tends to be the exception rather than the rule. Much more typical is Joachim Whaley's two-volume Germany and the Holy Roman Empire which limits its scope chronologically from 1493 to 1806 and Central Europe. It is telling that even with these limits, Whaley's two volumes run collectively to around 1500 pages. Wilson limits himself to only a 1000 pages in his history, and he organizes it thematically to better get a handle on the complex history of the HRE.

For the Middle Ages, Benjamin Arnold is probably your best English-language bet. There's Medieval Germany, 500-1300: A Political Interpretation. And if you really want to get into the nitty gritty of things, Princes and Territories in Medieval Germany.

The opening chapters of Thomas Brady, German Histories in the Age of Reformations, 1400-1650, offer a concise and reasonably clear picture of imperial reform efforts in the 15th century. Along with Hillay Zmora's State and Nobility in Early Modern Germany, which is a full-blown academic look at the issue, Brady describes the process by which the robber knights/lower nobility came more firmly under the grip of the princes and emperor. He also discusses the broader structural issues of the Reichstag and the relative strength of the cities vs landed nobility.