A primary source is an item that can be directly dated to the period that is being studied, eg. a manuscript, clothing. A reprint of a book two hundred years later should only be regarded as a primary source for the time that it was printed.
A secondary source is a professional discussion about a primary source, or historical commentary that was written by someone who was not present during the period or events that are being discussed. Most non-fiction books are secondary sources. Even medieval historians often wrote about events that occur centuries earlier, eg. Geoffrey of Monmouth