Depending on your definition, the Holy Roman Empire lasted over either seven hundred or a thousand years. Throughout this period the geographical borders constantly changed, and so did the institution's role: sometimes it dominated Europe, sometimes Europe dominated it. These are my top books on the subject.
1. The Holy Roman Empire 1495 - 1806 by Peter H. Wilson
In this slim, but affordable, volume, Wilson explores the broad nature of the Holy Roman Empire and the changes that occurred within it, while avoiding unnecessary, perhaps even unfair, comparisons to 'successful' monarchies and the later German state. In doing so, the author has produced an excellent overview of the subject.
2. Early Modern Germany 1477 - 1806 by Michael Hughes
In this book Hughes covers the major events of the period through a chronology, whilst also discussing the possibility and nature of a 'German' culture and identity within the Holy Roman Empire. The book is suitable for general readers and students, especially as the text notes previous historical orthodoxy. The volume also has a nice reading list, but too few maps.3. Germany: a New Social and Economic History Vol 1 edited by Bob Scribner
The first of a three part series (volume 2 is equally good, covering the period 1630 - 1800) this book presents several historians' work, some of which is usually available only in German. The emphasis is on new interpretations, and the text covers many issues and themes: this book will thus be of interest to all.4. Charles V: Ruler, Dynast and Defender of the Faith by S. MacDonald
Written as an introduction for mid to higher level students and general readers, this book is concise, clear in its explanations and modest in price. The text has been divided into numbered sections to allow for easy navigation, while diagrams, maps, reading lists and sample questions - both essay and source based - are scattered liberally throughout.5. Emperor Maximilian II by P. Sutter Fichtner
Fellow emperors such as Charles V may have overshadowed Maximilian II, but he is still a prominent and fascinating subject. Sutter Fichtner has used a large range of sources - many little known - to create this excellent biography, which examines Maximilian's life and works in an eminently fair and readable manner.6. Society and Economy in Germany 1300 - 1600 by Tom Scott
The use of 'Germany' may appear anachronistic, but Scott is dealing with the German speaking peoples of Europe, located largely within the Holy Roman Empire. As well as discussing society and economy, the text also covers the changing political structure of these lands, both geographically and institutionally; however, you will need an existing background knowledge to fully understand Scott's work.7. The History of the Habsburg Empire 1273 - 1700 by J. Berenger
Part one of a large two part study on the Habsburg Empire (the second volume covers the period 1700 - 1918), this book focuses on the lands, peoples and cultures ruled by the Habsburgs, the perennial holders of the Holy Roman Crown. Consequently, not all of the material is explicitly connected to the Holy Roman Empire, but everything is relevant and clearly presented.8. The Thirty Years War by Ronald G. Asch
Subtitled 'The Holy Roman Empire and Europe 1618 - 1648', this is one of the better books on the Thirty Years War. A modern examination, Asch's text covers a range of topics, including the crucial conflicts in religion and state. The book is aimed at mid to higher level students, balancing straightforward explanations with historiographical discussion.9. Germany Under the Old Regime 1600 - 1790 by J. G. Gagliardo
A relatively straightforward history text, Germany Under the Old Regime offers a narrative examination of the society, culture, politics and economics of the many Germanic states within the Holy Roman Empire, revealing an apparently divided, but in reality cohesive, German identity.10. War, State and Society in Württemberg 1677 - 1793 by Peter H. Wilson
Using the politics and society of Württemberg as a case-study, Wilson explores - and often heavily reinterprets - the nature of the 'German' principalities and their relationship with both Europe and the Empire. As expected, much of the text relates to Württemberg but the conclusions Wilson draws, and the material he offers, will be of interest to a wider audience than the title suggests.