Spent some time this morning looking into the clues of the Spalding Enigma, when I finally got into that section of the Howe story -- who first published the materials on Spalding and Rigdon.
The Wikipedia article seems oddly biased toward the apologetic LDS position.
Broadhurst's website has a reconstruction that lacks the footnotes (sigh) but makes sense as a narrative, if the omissions from the official LDS history that they record are indeed correct. Specifically, the claim is that Smith and Cowdery met before 1827 (in 1822) and that Rigdon knew Spalding and his work and was instrumental in the publication of the Book of Mormon, faking a conversion experience when Pratt and Cowdery came to missionize.
A separate group of guys that are friends of Broadhurst and did a lot of the legwork of the Spalding-Rigdon theory wrote a book about the enigma in 2005; the have a website for their efforts.