I always liked the fact that Joshi uses the first person occasionally. There are a few reasons why, I think, one of them being that it personalizes the writer of the biography, and allows the reader to thereby feel closer to the subject (Lovecraft). Most biographies have that false screen behind which the writer is hidden (sometimes by the editorial "we", which is arguably much sillier than using "I"), therefore projecting themselves as the Truth. By using the first person, Joshi is removing that screen, and saying "this is what I was able to find, and what I think it might mean". I like the speculative nature of Joshi's authorial voice, it makes me trust him in sections of the book when highly informed speculation is needed; he will run through the facts and available documentation, then tell the reader what, after all this research, is his personal gut feeling on the matter. The Lovecraft biography is such a well told story, and Joshi never actually interjects himself on a really personal level; he never presents his own personality to the detriment of biographical clarity. He only uses the "I" when he is making a scholarly summation based on the facts.
To criticize the use of the first person is the kind of attack a rigidly orthodox academic would make. It might have some basis in the realm of doctrinal correctness, but in the real world, especially if we judge the biogrpahy on its own merits (which are many) it really doesn't damage its effectiveness; in fact, just the opposite is true, in this reader's estimation.