Philosophy
Cambridge companion to early Greek philosophy
Cambridge companion to early modern philosophy
Cambridge companion to feminism in philosophy
Cambridge companion to German idealism
Cambridge companion to Greek and Roman philosophy
Cambridge companion to logical empiricism
Cambridge companion to medieval Jewish philosophy
Cambridge companion to medieval philosophy
Dictionary of untranslatables: a philosophical lexicon is a dictionary of terms of problematic translation language to language or culture to culture
Encyclopedia of medieval philosophy : philosophy between 500 and 1500
Internet Encyclopedia of Philosophy
Metaphysics : the key concepts
Oxford Companion to Consciousness
Oxford Companion to Philosophy
Oxford encyclopedia of philosophy, science, and technology in Islam
Oxford Dictionary of Philosophy
Routledge Encyclopedia of Philosophy Online. A more advanced treatment.