Let me clarify that I love Sherlock Holmes, and his stories. But this one... Not the best. Let me explain why. This story shows more the man than the detective; we see him as a human being, who also makes mistakes, and unfortunately, this time, that mistake costs the life of his client.
Personally, I think this story had great potential, but it isn't fully seized. Sherlock's amazing deduction skills barely appear, for starters, and it is the element I enjoy the most in his cases. And on the other hand, there is the mention of the Klu Klux Klan, and with such thing, I think that there's a lot of other ways to tell more complex stories; I mean, with such an element (with both historical background and consequences), the case could have been a lot deeper, because it presents a huge range of possibilities, that, in my opinion, aren't fully developed.
It could have been better. Still good, don't get me wrong, but not as good as other cases I've read, in which the author's genius can be read in both Sherlock's words and actions.