A primary source is any record contemporary to an event or time period. Primary sources may be written, oral, visual or physical. They may be intentional (created for posterity) or unintentional.
In the case of Martin Luther, primary sources are mainly the written works of Martin Luther and can also be other sources written during Luther's times.
There are certain words that appear in the subject headings of items in MERLIN that constitute primary sources. The most important of them is sources, but there are others.
In the simple keyword search box, you can put such words after SU: in order to specify the SUBJECT field.
ex.: crusades and SU:sources
In the advanced keyword search, you can put your search term(s) in the top box, and in the second box, put the word(s) you're using to locate primary sources in the next box, and change the drop-down menu to Subject:
Keywords that will help you find primary sources:
Depending on the period being studied, it can also be helpful to limit your search by publication date.
More keywords identifying primary sources in Library of Congress Subject Headings.
The Center for Research Libraries in Chicago is a treasure trove of rare primary sources, which they loan to member libraries on generous terms. CRL's catalog shows you some key items available for loan. on the subject of the Reformation.
Sources on this topic are often in languages other than English.
If you locate an item you wish to request, use ILL@MU.