My impression (possibly quite inaccurate) is that a lot of what's available in English on the Shinsengumi has come out of fan communities. I haven't looked into the subject in years, but there wasn't much scholarly material available at all in English. There was a Yahoo group for English fans collecting and exchanging fragments of information, translations and so on. (I think they may have started an LJ community or a website or something of the sort. I ran out of time to read posts and got frustrated because I didn't have the basic, assumed knowledge to understand half of what was said on the list.)