Columbia Encyclopedia: Hartmann, Nicolai - Short biographical article, from the sixth edition of this reference work. - http://www.bartleby.com/65/ha/HartmannN.html
FOLDOC: Hartmann Nicolai - Concise biographical entry from this database. Includes recommended reading and links to related entries. - http://www.swif.uniba.it/lei/foldop/foldoc.cgi?Hartmann+Nicolai