Investiture Contest is the basis of the culminating section of Poul Anderson's The Shield Of Time. Conflict with the Pope over the appointment of bishops and the usefulness of a tame inquisition are mentioned in SM Stirling's Changed world. Stirling's Lady Sandra, like Anderson's Gunnhild, comes to a pragmatic political conclusion about the advisability of an alliance with Rome - or, after the Change, Badia.
Stirling and Sandra refer to Stalin asking how many divisions the Pope had. I know that Anderson makes the same reference somewhere but I can't find it.