I'm so tired of explanations that just say "Charlemagne... A bunch of states... Its confusing... Napoleon." Like I want a list of every single state, when each one began, when each one ended, all of their rulers, all of their alliances, etc.
I’m no expert on the HRE, but I know a good source when I see one and I would consider ‘The Holy Roman Empire: A Thousand Years of Europe’s History’ by Peter H. Wilson. It’s well-sourced and footnoted, it’s detailed, it’s well-written, the author is knowledgeable and it’s accessible for a newcomer to the subject (like me). It’s a weighty tome, as it must be for such a massive topic. It is mostly a narrative history, which just personally is my favourite kind, but naturally it has thematic discourses where appropriate. It’s a very good book.