"Periodical/trade journals" refer to <em>scholarly articles, </em>which means that they are written by<em> certain experts</em> in particular industries. This also means that they are professional publications and may also include<em> newspapers, journals, etc</em>.
In order for Jennifer to know more of the person's work, she has to use the <u>periodic/trade journal</u>. This means that the sources that she will be referring to are <em>similarly professional journals </em>because the writer is an expert.