Meg,
There is some material on this in:
E. Vodola, Excommunication in the Middle Ages (Cambridge, 1986).
Some absolutions devolved eventually to the Apostolic Penitentiary:
K. Salonen & L. Schmugge, A Sip from the Well of Grace (Washington, DC, 2009).
There are lists of Reserved Cases, including those reserved to the pope, attached to some pastoral manuals.
Tom