The Oxford Bibliographies Online series is a first stop for acquainting yourself with the current state of the discipline in a given field. The series offers discipline-focused, online guides to the essential literature in subjects across the humanities and social sciences. Each subject module includes a full set of entries with introductory essays written by scholars in the field. These entries range from general overviews to highly-specialised themes. Each entry provides a synoptic, bibliographic guide to the key literature and the most useful online resources in a given area of research.
Listed below are bibliographies most useful to historical studies, but there are many others which may be relevant (international studies, Latino studies, etc.).
Encyclopedias are another important reference resource, especially when approaching a time period not in your direct area of research.
Not all encyclopedias have been translated into an online format, so check OneSearch and search for encyclopedias in the subject line followed by the century or the era (Victorian, Edwardian, etc.) as a keyword search to see if there are more examples in print similar to the last entry in this list.