Important general resources for researching the Middle Ages, as well as sections on religion, women, and specific countries.
Articles in scholarly encyclopedias usually present a good overview of the topic and identify the current issues, approaches, and scholarship relating to that topic. They often have good bibliographies that can lead you to other books and articles on the topic.
Bibliographies are great for finding out what has been written/is available on a specific topic.
The Cambridge Histories are scholarly, highly respected volumes of essays on the histories of countries and subjects. Each chapter is generally accompanied by an extensive bibliography. Among the volumes currently available online:
Topical dictionaries may have shorter entries focusing on people and terms.