Selected books, journals articles, and reference books published by Cambridge University Press.
Online encyclopedias, dictionaries and other reference books on all subjects, published by Oxford University Press.
Note that in CrossSearch, many topics, events, groups, etc., have a "Research Starter" entry at the top of the results. This is a great way to get started, often with timelines, jumping-off points for other references, and important terms and themes in that topic.