PhilPapers is a comprehensive index and bibliography of philosophy maintained by the community of philosophers. It includes content from journals, books, open access archives, and personal pages maintained by academics.
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Cambridge Companions [via Cambridge Collections Online] contains full text books and chapter extracts. It includes Cambridge Companions in Literature and Classics; the Cambridge Companions in Philosophy, Religion and Culture; and the Companions in Music.
Stanford Encyclopedia of Philosophy is a reference work on philosophy maintained by Stanford University. Each encyclopedia entry is authored and updated by an expert or group of experts in their field.
Oxford Handbooks Online is a collection of the best Handbooks areas across many different subject areas. The Handbook series contains in-depth, high-level articles by scholars at the top of their field.