Russell Anderson, The Spaulding Theory Debunked
Matthew B. Brown, Solomon Spaulding and the Book of Mormon
Dean C. Jessee, 'Spalding Theory' Re-Examined
Lance D. Chase, "Spaulding Manuscript" (Encyclopedia of Mormonism)
L. Ara Norwood's review of Vernal Holley, Book of Mormon Authorship: A Closer Look
Matthew Roper, “The Mythical "Manuscript Found" (review of Wayne L. Cowdery, Howard A. Davis, and Arthur Vanick, Who Really Wrote the Book of Mormon? The Spalding Enigma)
Matthew Roper, Paul J. Fields, and G. Bruce Schaalje, Stylometric Analyses of the Book of Mormon: A Short History
Matthew Roper and Paul J. Fields, The Historical Case against Sidnney Rigdon's Authorship of the Book of Mormon
It should also be noted that one can refute the various theories and “spins” on the Spalding theory using just non-LDS and anti-LDS sources. For instance:
John Hamer, "The Spaulding Fable" (a spoof-piece that also highlights the inanity of the "missing mauscript" theory)
Sandra Tanner, The Solomon Spalding Theory
Dan Vogel Discusses the Spalding/Rigdon Theory (here, Dan Vogel systematically destroys the purported evidences for the different "spins" on this theory)