@farmerman,
His first letter is called the "Dear Boss" letter. It seems to me that based soley on that use of the word boss, and some ineptitude in composition and spelling, it was assumed that this was an American (note that the English are making an assumption that the Americans are less likely to write "properly"). But in that same letter, he uses "shant" [
sic] rather than "won't," which would be more likely for an American. He also speaks of "the proper red stuff" (meaning blood), which while not incomprehensible to an American, is not a locution an American would be likely to use.
There were hundreds of letters and notes received, but only three which have since been taken seriously by any author with a plausible line. The "Dear Boss" letter is the one which introduced the sobriquet "Jack the Ripper." In another letter (or perhaps it was the post card), he uses the name "Saucy Jack." Once again, while not unknown to Americans, it is not as likely be used by Americans. The Metropolitan Police believed that at least two of these three were the products of a local journalist, and said that they were certain that they knew who that was. Whether or not that were true, it is possible that, believing it, they intended to warn that journalist off creating hoaxes.
The post card was considered (and to this day, still is) conclusively from the Ripper, because it was said to contain a detail which no one could have known in advance. However, the Metropolitan Police pointed out that, however it was dated, it was postmarked 24 hours after the murder which it purports of describe in advance, and that details of the murder were quickly known in the neighborhood. Those who still claim it is conclusive evidence of the authenticity of the note are obliged to ignore the post mark. In a period of my life when i was fascinated by "true crime thrillers" (the obsession soon left me, thank Dog), i read several books on the Ripper. There's not much to choose between them. They fall generally into two categories. Those who claim to know who it was, and who therefore ignore evidence to the contrary, and also don't canvass other theories. And there are those who just survey the events and the evidence, and the subsequent historiography of the Ripper incidents. The first category i would toss aside immediately--no one knows who it was, and i don't have any interest in someone's attempt to cozen me. The second category is interesting, but once you've read two or three of them, there's not much point in reading any more.
The fictional career of Sherlock Holmes spans a period from 1878 to 1916, and some people have wondered "aloud" (that is to say, in print), why Conan Doyle never turned his pen to the murders. I would say that Doyle had a proper respect for the victims, and the sense not to involve himself in that mare's nest.