Background or "reference" sources are a great place to start your research. Reference works include bibliographies, scholarly encyclopedias, dictionaries, handbooks, and other sources that provide overviews of topics and suggestions for further reading.
The Oxford Research Encyclopedias offer long-form overview articles written, peer-reviewed, and edited by leading scholars.
Link to the general collections of Cambridge University Press, including the Cambridge Histories and Cambridge Companions.
Oxford Bibliographies
An extensive collection of annotated bibliographies often pointing to both primary and secondary sources. You can browse by subject areas or keyword search for relevant articles. Sometimes not quite as up-to-date as the Oxford Research Encyclopedias, but still a very good resource.
Oxford Handbooks Online
Lengthy chapters in the Oxford Handbooks usually provide helpful overviews of scholarly topics and historical literature, along with suggestions for further reading. A number of the handbooks may be helpful depending on your research topic.
A large collection of resources. The Wiley Companions are often very useful. Navigate to "Humanities" and then "History" in order to find a detailed listing of titles by subfield.