Is it worth acquiring the Selected letters collections edited by Derleth and others at all?
If you refer to the Arkham House
Selected Letters, and you can afford to do so... most definitely! Though they definitely have flaws (peculiar editing at times, in one case resulting in inclusion only the greeting and closing of the letter, with none of the text; quirky excisions or inclusions, such as printing two letters which are virtually identical with one immediately following the other; and occasional misreadings of Lovecrat's handwriting, resulting in some rather peculiar constructions and words), nonetheless they are a major milestone when it comes to studying Lovecraft, and present close to 2000 pages of his correspondence, covering nearly every subject imaginable; they are a wonderful look inside the man with all his complexities, strengths, and foibles.
As for the more recent letters volumes -- those put out by Hippocampus Press, Night Shade Books, Wildside Press, University of Tampa Press, and the like -- again, definitely. These volumes are much more lightly edited, presenting nearly the entirety of his letters intact, which provides even more rich information on how closely interconnected his life, thought, and writings were -- amazingly so, as a matter of fact. Joshi is planning to put out a complete edition of all of his surviving correspondence (which he estimates will take up 25 volumes or more), but this is a long-term project, and the vicissitudes of fortune being what they are, I do not hesitate in again recommending the Arkham House volumes if you can afford them.
In addition to the letters themselves (including, occasionally, facsimiles of the letters) they also include numerous photographs of HPL, his colleagues, his wife, reproductions of various pieces of his artwork, etc. And
Lovecraft at Last, by HPL and Willis Conover, is a genuine gem. Though Conover edited the letters in such a way that the book is presented as a series of conversations between the two of them, Joshi has included an appendix at the back identifying exactly what letters each bit of text is from; and this book, even more than the Arkham volumes, is rich with reproductions of the letters, artwork, photographs, HPL's work in fanzines of the day, and numerous other items of interest. It is also perhaps the most intensely personal of the letters volumes, and a deeply moving memoir of a young man's mentor in the final months of his life.
HPLA - Lovecraft at Last
I would also highly recommend
Lovecraft Remembered, edited by Peter Cannon, as it is the best single collection of reminiscences, memoirs, estimates, and the like, ever assembled:
HPLA - Lovecraft Remembered
