Well, if you want some criticism:
The ending is pretty well telegraphed from about halfway through.. really, from about when they start reading the book. Of course, the details of the ending aren't clear until Wainswright's fate is described, but one gets the gist.
Note: Lovecraft does this too, so it's to be expected in pastiche.
Also, the repetition of the "Yes, I thought so." phrase (or some variation thereof) is kind of tedious, but so are many old men, heh.
I thought it was a very good story, and better than some published Lovecraft tribute stories I've read, though. Those two points are just something I figured I'd mention in the name of providing criticism. They're really sort of minor.