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Mmm, this is a bit of a tricky one. Firstly, what level is this aimed at? Are you in secondary level education or at university? I presume you're looking for English-language resources if the project is from an English class? Is the emphasis/grading more on your presentation and use of English, or on the facts and the history themselves? Do you have access to an academic library, or are you relying more on the internet/easily accessible resources?

I'll hopefully be able to give you a more detailed answer later if you give a little more information on your project, but for the moment I'll recommend you two 'classics' which I would see as introductory level for university students: R.W. Southern's Western Society and the Church in the Middle Ages and Colin Morris' The Papal Monarchy. Both are accessible and pretty straightforward intros to the basics.