From what I've read so far the book's main value for me is in Joshi's giving the date when Lovecraft wrote each story described; this is useful in getting a sense of the development of his career, and the point at which specific ideas entered (this latter point being the main reason I'm reading the book). Joshi's judgments of the stories' merit or otherwise are typically idiosyncratic. In essence Joshi divides said stories into "the Lovecraft Mythos", of which he approves, and "the Cthulhu Mythos", which he does not. Given his years of working on Lovecraft, it's disconcerting that at one point he summarizes the plot of THE DREAM-QUEST OF UNKNOWN KADATH, and gets it wrong.*
For the sake of determining whether a story is or is not part of the Mythos, Joshi singles out several iconic features that tend to distinguish Mythos tales, noting that not all need be present in each story: "fictional New England topography" (Arkham, Dunwich, Innsmouth), forbidden tomes (the NECRONOMICON), extraterrestrial god-like entities, cosmicism, and (sometimes) a scholarly narrator (p.16–18). I'd drop 'cosmicism' from the list but otherwise think this is a sensible approach.
For Joshi's critique of said stories, by Lovecraft and others, he lays down several principles by which to judge each individual story (p.12):
"intrinsic literary merit"
"skillful and effective prose style"
"competence in the execution of the plot"
"a . . . distinctive message about human life and the cosmos"
This list is of particular interest because I find Lovecraft's work distinctly lacking in just those features. I enjoy reading Lovecraft's stories the same way I've come to enjoy Godzilla movies, but don't think either transcends the category of pulp fiction. Joshi, however, sees a literariness that doesn't register for me. So reading his book I do get to see HPL's work through his eyes, albeit somewhat skeptically. We'll see if he wins me over as I continue to make my way through the book.
--Little Rock Clinton international airport.
*Joshi writers "Randolph Carter seeks to confront Nyarlathotep and demand the return of the 'sunset city' of his dreams" (p.44). In fact during his quest to find "the mild gods of earth" Carter takes great pains to avoid the Other Gods in general and Nyarlathotep in particular, correctly guessing that this would spell disaster.