You can use citation indexes to count how many times a source has been cited, which is one measure of scholarly influence. You can also use citation indexes to see which books/articles have cited the source in question.
By tracing a source forward and backward through time, you are tracing a thread of the scholarly conversation on this topic.
A scholarly book review offers both an evaluation of the book and a sense of how the book fits in with other academic works on the topic. You can use academic book reviews to identify other relevant sources.
The database America, History and Life contains book reviews on US history (along with scholarly journal articles and other sources). The database Historical Abstracts offers book reviews, articles, and more for non-US history.
Doctoral students go through an exhaustive literature search when writing their dissertations. They also tend to work on new questions, sometimes ones that have received very little attention from scholars before. You can obtain many of these dissertations and benefit from all the bibliographic treasues and new ideas. Check ProQuest Dissertations & Theses to identify dissertations on your topic.
To find books that are not owned by the TriCo libraries, search by keywords and/or subject headings in WorldCat, and make interlibrary loan or EZ-Borrow requests. Whenever possible, try to make your interlibrary loan requests early in the semester, to ensure that the books arrive with enough time left to write your paper.