It isn t enough that the author sounds like an authority, or even that he or she has the credentials to prove it. Make the author prove his or her point, and if he or she cannot do so, do not blindly accept his or her word as truth. Continue your own research, and see if the evidence YOU find supports or rejects the author's hypothesis. The third step is the process of comparison. It is very difficult to develop an eye for good evidence and a good argument unless you know what skimpy evidence or a badly-constructed argument looks like.
Few books are completely error free, and by the same token, few books are completely without value.
When you exhaust the books available on your subject, broaden your search. For costume, for instance, after you ve looked at the available histories of costume, you could start looking at facsimile manuscripts and books on art and sculpture. You could start reading books on the textile industry so as to begin to appreciate the way fabric was made. You could look at period literature for references to what people wore. You could even start looking into household accounts to get an idea of how much fabric cost and what kinds of items were purchased each year.