General reference texts such as Dictionary of Literary Biography, Contemporary Authors, or Literary Research Center , etc. These are obvious sources of needed information. Letters and other personal documents. Sometimes the collected letters and other personal documents are published together in a volume put together by an editor. They are an important source of information about the writer's life, his or her reactions to reviews of works, facts and feelings about friends and associates, clues to what was on the writer's mind at any given time, etc.
Some libraries own special collections.
For instance, the Harry Ransom Center at the University of Texas owns thousands of files of writers' personal correspondence, etc., none of which is available anywhere else. Still, you should know that such collections exist, for you will often find references to them in your reading. Locate the books and articles and read them. Look at the bibliographies in each of these sources that you find. Notice any works that look as though you ought to have a look at them.