Lester Little's case study of Bergamo, Liberty, Charity, Fraternity: Lay Religious Confraternities at Bergamo in the Age of the Commune is a bit of a classic, inasmuch as certain monographs can become classics. I'd honestly recommend his other book, Religious Poverty and the Profit Economy in Medieval Europe, if one were to read merely one Lester Little monograph in their brief lifetime. For a more recent study, Frances Andrews has The Early Humiliati, which describes just that. I found that the author assumed the reader had a certain foreknowledge with that volume however. If you were looking for a more generalized discussion, there are several good surveys of the "medieval city" whose references I can dig out.

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