Gold Mining: Turning One Good Source into Many Good Sources
When you get a good source, seek your source’s sources. That writer has been working for you for years, and has assembled the sources you really want to find. In gold mining, this is called "high-grading the ore" to concentrate on processing stuff that already has been pre-selected as "the best." For instance, when panning gold, they don't just process all the sand in Summit County, Colorado, but rather they seek the lowest spots in gravel banks located in rivers running out of known gold-bearing geological formations. The river has been patiently and powerfully crushing and sorting rocks for millions of years, and it always puts the gold in the same sorts of places, on the bottom, jammed in cracks in the river's bed, because gold is heavier than other elements. Similarly, the professional scholar's book arises out of a similar crunching and testing and sorting of the scholar's predecessor sources, and they're all stacked in neat little piles in the source's annotations and bibliography or Works Cited section. Read the titles with some discernment, check the index and find where and how they're being referenced, and go for the ones described as "insightful," "indispensable," "amazing," and "essential," etc.
Also, when you get a good source, search again under your source’s name. Critics develop methodologies that work on many authors, and an essay on Hemingway might work well as a source for terms of art or logical strategies that also will work on Hawthorne. Also, they tend to follow the data they've created on one topic to further conclusions expressed in more books and articles, sometimes collected as essays in topical collections not specifically edited by them. The Humanities Index is a great way to spot those chapters. The main principle is that your source, like the gold miners' river, keeps producing results all along its career, and to stop with the first one you find might be the reason why you're not rich! The treasure goes to those who persevere intelligently and who never stop too soon.