While more can always be said on the topic, I hope my following previous posts will be a bit of use to you:

• Did the 'Investiture Controversy' not begin over investiture?
• What are some good reading recommendations on the Investiture Controversy?

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But it seems to be something that has older roots and a long tail after the 11th century. I'd like to better understand that rivalry, its source, its main actors and its main up and downs.

I'd also recommend you to take following sub-topics also in consideration:

• From "Europe of Bishops" (Cf. Reuter 2011) - Local bishops, especially in Germany, as the third actor, and their relationships either with the Pope or the emperor (king) of Germany in the High Middle Ages.
• Have you heard of "Imperial Church System (Reichskirchensystem)"? This concept [as a distinct practice among the Germans] has been criticized by Timothy Reuter, but he does not in fact refute the significance of the close relationship between bishops and the ruler (especially Salian ones) itself totally (Reuter 1982). To give an example, German bishops were expected to contribute also to the military resources as well as lodging to the ruler.
• Institutionalization of the Church - To what extent the Pope was the de facto top of the hierarchy of the Catholic church, or what did he have or can use to communicate his instruction? Robinson's classical studies (see the second link) especially address this topic. Melve's new study also focus on the historical significance of this debate in the communication across Europe.
• "Crisis of Medieval Germany" - as formulated by late Karl Leyser (in about 40 years ago): As for Canossa, the shifting power balance within the kingdom of Germany in course of the early years of King Henry IV of Germany, played an important role. In English, not so many scholars (especially non-specialist) pay enough attention to this elements (in spite of excellent biography of Henry by Robinson).
• Formation of "Latin Christendom", defined primarily as [Europe as] a community of Christian peoples under the leadership of the Pope and his hierarchical organization of the Latin Catholic Church: Since two breaches between Pope Gregory and King (not emperor then) Henry IV, both parties (especially the first, Gregorian ones) tried to establish the more stable communication with diverse rulers (kings, nobles and bishops) across Europe, especially its fringe area (see also my previous post in: The year is 1066 AD. What is the extent of Western Christianity i.e. Catholicism and its offshoots?). The mobilization and the response to the papal call for the first crusade in the end of the 11th century should also be seen from this point of view.
• The battle on the "right world order"? - More than 80 years ago, German historian Gerd Tellenbach formulates the historical significance of so-called Investiture Contest in his influential classic (also translated in English). The most fundamental of his arguments is that the conflicts in fact had wider implications, among others, re-definition of the nature and border (relationship) between the secular and ecclesiastical rulership.

Add. References:

• Bartlett, Robert. The Making of Europe: Conquest, Civilization and Cultural Change 950-1350. Harmondsworth: Penguin, 1993.
• Cotts, John D. Europe's Long Twelfth Century: Order, Anxiety and Adaptation, 1095-1229, European History in Perspective. New York: Palgrave Macmillan, 2012.
• Reuter, Timothy. “The ‘Imperial Church System’ of the Ottonian and Salian Rulers: a Reconsideration.” The Journal of Ecclesiastical History 33, no. 3 (1982): 347–74. doi:10.1017/S0022046900026245.
• ________. "A Europe of Bishops: The Age of Wulfstan of York and Burchard of Worms." In: Patterns of Episcopal Power: Bishops in Tenth and Eleventh Century Western Europe, ed. Ludger Körntgen & Dominik Waßenhoven, pp. 17-38. Berlin: de Gruyter, 2011.