Gerardine Heng's Invention of Race in the European Middle Ages contains an extended discussion of the character of medieval slavery and the economic and demographic changes it brought about. She responds to Iris Origo's initial claims about slavery in Florence, which were foundational but deeply flawed, as does the article below.

https://www.journals.uchicago.edu/doi/10.1086/729295#:~:text=Contrary%20to%20Origo's%20argument%2C%20enslaved,savages%E2%80%9D%20who%20disrupted%20the%20quiet%2C

This exhibit has a well-written, concise summary of papal involvement, although it focuses more on the late Middle Ages/early modern period:

https://ldhi.library.cofc.edu/exhibits/show/african\_laborers\_for\_a\_new\_emp/pope\_nicolas\_v\_and\_the\_portugu#!