One of the best, fastest and easiest ways to find new resources is to consult the citations of a resource that you currently have. An excellent side effect of this is that you begin to see who the "big names" in your field are - the works that everyone cites. You want to read these works, too, or at least try to.
This is another reason why journals are so wonderful. A book can easily have hundreds of citations; selecting just a few to pursue is daunting. A journal article will typically have between five and two dozen, which is much easier to handle. If you can gather three or four journal articles on your topic, their bibliographies should indicate your authoritative works as well as other more recent work in the field which may interest you.
Peer review is a process used by publishing academics to try and keep the nonsense out of print. Articles submitted for publication are reviewed by the submitter's academic peers (hence the name). The reviewers are expected to be knowledgeable in their fields and to be able to detect and distinguish between novel thoughts and errant nonsense.
The articles in scholarly journals are typically peer reviewed. Peer review is not infallible - some MIT students made news not too long back when they submitted a fake paper to a conference and had it accepted. Although the standards for conferences may be lower than for journals, depending on the field, this was still pretty inexcusable. But in general, peer review gives you some confidence that other experts out there found an article to be worthwhile.
Books are not necessarily subject to peer review. Just because it's from a branch of a major publishing house doesn't mean it's true. If your book builds its case on primary sources (the obvious meanings of primary sources, not the occult ones) and peer reviewed articles, you're better off than a book based on mysterious artifacts of uncertain provenance and tenuous correlations.
Speaking of which...
Absence of Evidence Is Not Evidence
Just because no medieval text specifically refutes the presence of flying pink elephants does not mean that there were in fact flying pink elephants.
If you see a source relying on an absence of evidence (e.g., "Because there is no evidence that this is not true, it must be true"), be suspicious.
Watch Assumptions Becoming Facts
Good authors are very clear in stating that they are assuming ABC and proceeding from there. Bad authors spin a case of a "hypothetical" case for XYZ in Chapter 1, which you might or might not believe, but in Chapter 3 this "hypothetical" case is back and being stated as if it were a fact.
This is like going to a car showroom to browse and having the salesperson tell you that "of course you want a model with all the latest features." You actually aren't sure you want a car at all. Anyone telling you, strongly that you do want a car and you want this kind in particular is trying to sell you something.
Correlation Is Not Causation
This is a tricky one, and it is so tricky that the best authors will especially call your attention to it and discuss how it applies or does not apply to their work. When one thing follows or appears to follow another, we tend to think that one is the cause and the other is the effect. In fact, they may be random events or they may both be caused by some other, hidden cause.
Example: The people of Village Y all used an herbal remedy against the plague. No one died of the plague in Village Y. Ergo, the herbal remedy was effective. (Or the village was in a remote district and no plague germs ever came there.)