Beginning with dictionary and encyclopedia articles is often wise. The articles, especially in encyclopedias, have footnotes with suggestions for further reading that can help guide your research very efficiently.
Dictionaries and encyclopedias are reliable sources from authoritative publishers with articles written by well respected scholars in the field.
They provide good background Information and help you to identify key terms, dates, people, theories, etc.
Although there are more ecumenical print texts available that are specifically Roman Catholic, the ecumenical texts usually discuss the distinction between Catholic, Protestant, and (sometimes) Orthodox approaches to an issue or concept.