Broken links updated 26 June 2011
My three primary sources were the books In Search of Butch Cassidy by Larry Pointer (© 1977), Outlaws, A Quest for Butch and Sundance by Eamonn O'Neill (© 1997) and The Outlaw Trail by Robert Redford (© 1978), in that order — not because these were the best sources I could find, but because they were the only books I could get on this obscure subject from the local library service! However, going by these reviews of the various 'Harvey Logan' books available in America: http://www.crosswinds.net/~waynek/Books.html (from WayBack Machine), I do seem to have struck relatively lucky.
Larry Pointer's book is scholarly in tone, including comprehensive references to his original sources, and where accounts conflict I'll admit to being biased in favour of his version. However, much of the book is based around his wish to establish that an unpublished manuscript from 1934 — The Bandit Invincible, the Story of Butch Cassidy — is the autobiography of Butch Cassidy, who survived his alleged death in South America; a theory which in the years since 1977 appears to have gone out of fashion. And naturally enough, Harvey Logan is mentioned only when his path happens to cross that of Cassidy, the biographer's actual subject.
Eamonn O'Neill's book is a modern travelogue, with nuggets of actual historical fact nestling between recollections of the author's childhood, anecdotes of his travails in modern-day America, and vivid retellings of the events of the outlaw past. The latter are in fact highly readable, and influenced me more than I care to admit in my own characterisation and descriptions; but since this is not an academic work there are no specific references, and often no way of telling which details have been fleshed out by the author's imagination and which are the result of recent new research. It is interesting that in his bibliography O'Neill cites both Pointer's and Redford's books as reference material, with particular emphasis on Redford's!
Yes, this is the same Robert Redford who is a Hollywood actor... and whose main association with American historical research consists of having once played the part of Harry Longabaugh, the 'Sundance Kid'. But The Outlaw Trail is a serious piece of work, undertaken under the auspices of National Geographic magazine — and it is interesting precisely because, unlike the other two books, it does not concentrate solely on Butch Cassidy's activities, but gives an overview of outlaw haunts and local memories, together with (thanks to its illustrated magazine origins) a large number of period photographs and reproductions.
It did not occur to me for some time to attempt a search for 'Harvey Logan'/'Kid Curry' on the worldwide web. When I did so, I was not overly impressed by the quality of what I found there. At least one page was obviously a cribbed (albeit cut-down) copy of a page on a completely different site, and statements like '[in 1901] Hank Logan was not known to have ever been in any trouble with the law' did not inspire confidence, given that the author was all too evidently unaware that the eldest brother had died of pneumonia at an early stage in Harvey's career.
Most sites appear to be based on other sites, in an incestuous loop that doubtless entails its own Chinese Whispers effect, and very few authors cite any original sources at all. I can do no better than to quote Ramon Adams (Six Guns and Saddle Leather, 1969, cited by Larry Pointer): 'many chroniclers seem to delight in repeating early sensational and frequently untrue stories without any real attempt to investigate the facts'....
However, I did find some data that seemed useful. Bruce Logan http://freepages.genealogy.rootsweb.com/~blogan/logan3.html provided extracts tracing the Logan and Lee families across relevant census years — and also suggested the existence of a sister, Arda Alma, who remained behind in Iowa when the boys were sent to live with their aunt, married, and left descendants! Elizabeth Gibson http://members.aol.com/Gibson0817/KidCurry.htm appeared to merit attention, not only because her article had apparently been accepted for publication in what I assumed to be a relevant and reputable journal, but also on the basis that she actually listed sources (if only other authors knew how easy it was to give themselves an extra air of reliability...!) Sections of her article are clearly based on information from Charles Siringo's autobiography, an important document to which I had no access. Both she and Kerry Boren http://www.prospector-utah.com/curry.htm cover periods of Curry's life on which the Cassidy-centred accounts are naturally silent — those during which he was not running with members of the Wild Bunch. I have to admit, though, that she evidently takes a somewhat more romantic view of Kid Curry and his world than I felt able to do: as she herself puts it: "He fell into trouble by mistake, but could never shake the outlaw life."
I was interested to come across the article by Kerry Boren, since he had been responsible for contributing a lot of the historical research to The Outlaw Trail in 1978. He covers in considerable detail the circumstances surrounding Curry's captivity and trial in Knoxville, events which are barely mentioned elsewhere, and quotes extensively from original sources. However, I am inclined to take with a pinch of salt his uncritical recounting of traditions such as a childhood encounter for young Harvey with the infamous Jesse James, and the Andrew Duffy story in South America, along with his tendency to claim aristocratic ancestry for his subjects.