You might be wondering why a History 400 student would consult subject encyclopedias. The quick answer is that it can't hurt to make sure you have a good sense of how your research question fits in with other scholarly research that has been done--and the bibliographies at the end of encyclopedia articles are an efficient way to make sure you know who the scholars are who've been working on this topic.
Not sure where to look? Each of these online collections will introduce you to a wealth of dictionaries and encyclopedias.
If the people you are researching are American or British and dead (in Boolean logic that would be "American OR British" AND "dead OR deceased"), try the ANB or the ONB. The bibliographies at the end of entries will give you information about the location of that person's papers--in archives if unpublished, or in scholarly editions, if published.
Please remember that subject encyclopedias can be excellent resources for identifying primary and secondary sources. The list below is just a sampling of the available titles; if you can't find a subject encyclopedia relevant to your topic, please email Peggy Burge for suggestions.