[Melve 2015] and [Miller 2009] are historiographical articles that summarize the basic trend of research on the topic, mainly focusing Anglophone works.

As a single-volume book, [Cushing 2005] is the most recent standard book on the relevant field (Investiture Controversy), though her focus might be a little too broad for my taste (but especially suitable as a first-read for the topic) . On the other hand, [Rennie 2013] and his other academic articles put emphasis on the development of papal legates and canon law, tools of church reform in late 11th and early 12th century. [Cushing 2011] indeed handles the latter topic (canon law), but is probably too specific for non-specialists.

[Robinson 1990] is the standard classic of the topic, focusing mainly on the institutional as well as political development of the Papacy as a consequence of the conflict with German rulers and bishops. His biography of King/ Emperor Henry IV ([Robinson 1999]) is also a must-read, especially if you cannot easily read other books on him in German original. [Cowdrey 1998] provides a formidable summary of scholarship in this key figure in the Investiture Contest, too (Cowdrey had also translated and published the whole letter collection of Pope Gregory VII in bilingual edition).

My previous posts on the relevant topics, cited below, are also mainly based on Robinson's works as well as some newer research in German historiography like Claudia Zey, with references in the end:

• Did the 'Investiture Controversy' not begin over investiture?
• Walk To Canossa- Papal Power- What power did the Popes use to assert their authority as they started to do so on an international level, most notably during the walk to Canossa.

References:

• Cowdrey, H. E. J. Pope Gregory VII, 1073–1085. Oxford: OUP, 1998. DOI:10.1093/acprof:oso/9780198206460.001.0001
• Cushing, Kathleen G. Reform and the Papacy in the Eleventh Century Spirituality and Social Change. Manchester: Manchester UP, 2005.
• ________. Papacy and Law in the Gregorian Revolution: The Canonistic Work of Anselm of Lucca. Oxford: OUP, 2011. DOI:10.1093/acprof:oso/9780198207245.001.0001
• (Open Access) Melve, Leidulf. "Ecclesiastical Reform in Historiographical Context." History Compass 13-5 (2015): 213-21. https://doi.org/10.1111/hic3.12232
• Miller, Maureen. "The Crisis in the Investiture Crisis Narrative." History Compass 7-6 (2009): 1570-80. https://doi.org/10.1111/j.1478-0542.2009.00645.x
• Rennie, Kriston. The Foundations of Medieval Papal Legation. New York: Palgrave Macmillan, 2013.
• Robinson, Ian S. The Papacy, 1073–1198: Continuity and Innovation. Cambridge: Cambridge UP, 1990. https://doi.org/10.1017/CBO9781139167772
• ________. Henry IV of Germany 1056–1106. Cambridge: Cambridge UP, 1999. https://www.cambridge.org/core/books/henry-iv-of-germany-10561106/E78D02BD3184A401350F96B97D4FB2B1

(Edited): fixes a grammatical mistake.