I just ordered this book and think it will be interesting-
“Mediolanum - La Zecca di milano dalle origini a Desiderio, Re dei Longobardi (IV sec. a.C. - 774 d.C.)” by Alessandro Toffanin (2014)
It covers a wide range of period from the Roman Republic until the Langobards. Though I don’t know how useful it will be, it does cross refence other sources for each coin, like it shows in the picture, Postumus coins are given a reference for RIC, Elmer, Cunetio hoard and Normanby hoard, if applicable. There are quite a lot of bronze from Milan during the 3rd century, from Gallienus to Aurelian; but when Constantius II reopened the mint, the mint struck only gold and then gold and silver for later rulers like Valentinian I and Valens.