What type of audience is the author addressing? Is the publication aimed at scientists, professionals or a general audience?
Is this source too elementary or basic, too technical or applied, too advanced, or just right for your needs?
You are more likely to retrieve documents written for the appropriate audience if you start off in the right database. For instance, a search in Web of Science will only yield articles from highly reputed academic journals, whereas a search in PubMed or CAB Abstracts will also yield articles from professional journals.