Von Pastor's History of the Popes from the Close of the Middle Ages is a must read for a serious student of Church history. You probably already own a copy of Fr. Denzinger's "The Sources of Catholic Dogma" as well.
I have no idea where you would find a physical copy of Von Pastor's series though. There was a complete set in the Yale University library. I know that it exists in its entirety (I believe) online as well.
Complete sets of 40 volumes can be found to buy online - abebooks.com has one set for example - but priced over $3,000.