He is credited as the discoverer of Manic Depression (now known as Bipolar Disorder) and Schizophrenia (which he called 'dementia praecox').
Krapelin was a colleague of Alois Alzheimer, and co-discoverer with Alzhemier of Alzheimers Disease.
See http://www.uni-leipzig.de/~psy/eng/kraep-e.html for a biography of Kraepelin, and http://www.med.uni-muenchen.de/psywifo/Kraepelin.htm for a bibliography of works on and by Kraepelin [in German].