I've dealt with specific allegations of plagiarism (Spaulding Theory, etc.) on my page about Plagiarism and the Book of Mormon. This page has long cited information about the limited supply of books in the Manchester/Palmyra area, but recently I ran into more relevant information about Harmony, Pennsylvania, where most of the Book of Mormon was actually translated. It looks like there wasn't even a library there, making it even more of a literary vacuum than Palmyra. This insight comes from John Welch's chapter, "Was There a Library in Harmony, Pennsylvania?" in Pressing Forward with the Book of Mormon, ed. by John W. Welch and Melvin J. Thorne (Provo, Utah: Foundation for Ancient Research and Mormon Studies, 1999, pp. 283-284), where we read:
Harmony was a small town on the border between the states of New York and Pennsylvania. The region was very remote and rural. Recently we asked Erich Paul [Erich Robert Paul apparently is the full name of the author of the previously cited article, "Joseph Smith and the Manchester (New York) Library"] if he had ever explored the possibility that any libraries existed around Harmony in the 1820s which Joseph Smith might have used. He responded: "In fact, I checked into this possibility only to discover that not only does Harmony and its environs hardly exist anymore, but there is no evidence of a library even existing at the time of Joseph's work." Accordingly, those who have considered western New York as the information environment for the Book of Mormon may be 120 miles or more off target. One should think of Joseph translating in the Harmony area and, as far as that goes, in a resource vacuum.
Even if Joseph had wanted to pause to check his details against reputable sources, to scrutinize the latest theories, to learn about scholarly biblical interpretations or Jewish customs, or to verify any Book of Mormon claims against the wisdom or theologies of his day--even if he had wanted to go to a library to check such things (something he showed no inclination to do until later)--there simply was no library anywhere for him to use.
Critics can continuing scouring the 19th century for scraps of information spread across American and European cities to "explain" various fragments of the Book of Mormon, ignoring the improbability of access to such documents for Joseph Smith. In doing so, they don't come anywhere close to meeting the burden of proof or even the burden of plausibility for their hypothesis.
And if we are to believe that a proposed source was plagiarized, their ought to be stronger evidence of derivation than chance patterns of a few related words of concepts. If Spaulding or anyone else is to be seriously considered as a possible source for the Book of Mormon, the evidence for derivation should be much stronger than the chance parallels we find in the "impossible" source, Leaves of Grass by Walt Whitman, where my analysis shows parallels far more impressive than those that the Tanners and others point to as evidence of plagiarism for sources that came before the Book of Mormon. Again, the critics simply haven't provided any reason to accept their case.