Sticking with English recommendations, and more of a political focus:

• Benjamin Arnold, Medieval Germany, 500-1500: A Political Interpretation (2003)
• Thomas Brady, German Histories in the Age of Reformations, 1400-1650 (2009) - read for the first chapters on 15th century imperial reforms
• Benjamin Arnold, Princes and Territories in Medieval Germany (1991) - dense but worth it in the end
• Robert Bartlett, The Making of Europe (1993) - much of this is about HRE/German expansion eastward
• David Abulafia, Frederick II: A Medieval Emperor (1988)
• Uta-Renate Blumenthal, The Investiture Controversy: Church and Monarchy from the Ninth to the Twelfth Century (1988)
• Bjorn Weiler, Kingship, Rebellion and Political Culture: England and Germany, c.1215–c.1250 (2007) - comparative, but puts it to good use
• Tom Scott, Society and Economy in Germany, 1300-1600 (2001)

Gerd Althoff is good on the Ottonians and I believe his book on Otto III has been translated into English. For Hanna Vollrath on the Salians in English, I think you have to go to the New Cambridge Medieval History volume on that era--but reading the HRE chapters in those books wouldn't be a bad idea, although they're a bit like drinking from a fire hose.

Let me know if you're interested in more social/cultural/religious history. A lot of my reading there is unfortunately really specialized, but I can pick out a few that seem to comment on something more than an individual monastic house. ;)