Combining the best features of an annotated bibliography and a high-level encyclopedia, this is a great resource to learn more about a topic and narrow it down.

PhilPapers is a comprehensive index and bibliography of philosophy maintained by the community of philosophers. This is a great resource to use to learn more about a topic and then find sources about that topic.

Provides online access to the back issues of a multidisciplinary array of journals. Coverage includes the arts and humanities; the social and political sciences; mathematics and the hard sciences; and business and economics.

Oxford Academic has a very large selection of Philosophy eBooks published by Oxford University Press.