Oxford's Very Short Introductions series "offers concise and original introductions to a wide range of subjects - from Aesthetics to Buddhist Ethics, from Derrida to Wittgenstein. Each volume provides an authoritative and engaging assessment of a concept, field, or body of work, drawing out the central ideas, themes, and approaches." (from Oxford UP)
Philosophy Includes the Cambridge History of Medieval Philosophy, Cambridge History of Jewish Philosophy, Cambridge History of Eighteenth Century Philosophy, The Cambridge history of Philosophy, 1870-1945, The Cambridge History of Hellenstic Philosophy, etc.
Updated regularly. Provides instant access to the renowned texts of the Cambridge Histories series. All the available volumes are grouped into broad topics for quick and easy browsing (see the list below), or the entire series can be searched using the basic or advanced search features. Specific histories consist of either one volume or a numered series and cover all major regions of the world (i.e. Cambridge History of Africa, Cambridge History of Latin America).
Internet Encyclopedia of Philosophy - Open access publication with peer-reviewed entries on concepts and figures in Philosophy by experts. Somewhat less comprehensive (and critical) than the Stanford Encyclopedia.
Stanford Encyclopedia of Philosophy - Wonderful open access publication with up-to-date entries on concepts in Philosophy by an expert or group of experts in the field. All entries and substantive updates are refereed by the Board of trustees
Oxford's Very Short Introductions series "offers concise and original introductions to a wide range of subjects - from Aesthetics to Buddhist Ethics, from Derrida to Wittgenstein. Each volume provides an authoritative and engaging assessment of a concept, field, or body of work, drawing out the central ideas, themes, and approaches." (from Oxford UP)