The most convenient starting point should be the literature list of NCM: New Cambridge Medieval History, ii (1995)'s Delogu's article on Italy in Langobard and Carolingian Period: https://www.cambridge.org/core/books/abs/new-cambridge-medieval-history/lombard-and-carolingian-italy/3861DC8BFF092A37BD6FEF4D7EE5C342

You can see the name of (Stefano) Gasparri (must read, if you can read Italian), Hlawitschka and Houben and their works in the list linked above.

Other possibly good introduction of historiography are:

• Davis-Secord, Sarah C. "Medieval Sicily and Southern Italy in Recent Historiographical Perspective. History Compass, 8 (2010): 61-87. https://doi.org/10.1111/j.1478-0542.2009.00651.x
• Schoolman, Edward M. "Aristocracies in early medieval Italy, ca. 500–1000 CE." History Compass. 2018 (2018); 16:e12499. https://doi.org/10.1111/hic3.12499

Anyway, I'm afraid that you have to check the following proceedings by yourself sooner or later: IX. Atti del 9° congresso internationale di studio sull'alto medioevo: « Il Ducato di Spoleto» (Spoleto, 27 settembre-2 ottobre 1982).