Well, after a day preparing for having friends around for dinner yesterday, I wasn't able to get back to this until today.
(Full disclosure: I'm not all that concerned about spoilers here, given the unique nature of the tome and the knowledge that the only other person I know to have a copy of it has already read it - and taken way better notes.)
The good news is I was right - I guessed the killer. The not as good news is that I did it almost immediately. The entire thing hinged on a false assumption made at the start, around page 15 or so. If the reader picks up on that false assumption, the rest of the file is really rather extraneous. In fact, I was more than a little nervous about looking at the solution because I felt like I had to be missing something. It turns out I was, but only a handful of smaller clues that supported the answer. I'd have really liked having to rely on those clues; solving the mystery would have been a lot more fun if I'd had to search them out. As it was, I was so certain about the twist, I didn't look very hard at the evidence.
There's one caveat to my gripes though; if I'm being objective (and I try to be), I have to say that this mystery file probably suffers to a greater degree because I've read another - one written later and by another author - first. That one, File on Fenton and Farr, was far more intricately plotted, and strung the actual clues out far longer, than Murder Off Miami, leaving me tossing theories around until almost the very end. I got that one right too, but I had to really work for it, and if anything, that's what disappointed me about this one.