Books about the corruption of the Catholic church during the Middle Ages?

I need some sources for a research paper on the corruption of the Catholic church during the Middle Ages. Do you know of any books about this topic? Also - any legitimate websites about this?
Update: Again people, this is for a RESEARCH paper. For some of you: this means relax. For others: this means I need legitimate, non-fiction sources.
Update 2: Again people, this is for a RESEARCH paper. For some of you: this means relax. For others: this means I need legitimate, non-fiction sources.
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