Good question -- here are a few reasons:
Several databases can be good beginning points with introductory overview essays on your topic and some include bibliographies of the principal recommended secondary and primary sources. These can be good starting points for thinking about your topic and identifying the key recommended works.
Reference sources published by Oxford University Press. Dictionaries, subject reference works, language reference sources and key titles in the Oxford Companion series. Some examples are: the Oxford-Duden German dictionaries, A Dictionary of Psychology, A Dictionary of Zoology, The Oxford Dictionary of Idioms, A Dictionary of Writers and Their Works, Oxford Companion to Western Art, Oxford Companion to U.S.History, Oxford Companion to Music, Oxford Companion to Philosophy, and The Encyclopedia of Popular Music, etc.
Sometimes the best place to start is a subject specific encyclopedia. All titles below can be accessed online.