I don't have all the collections he did for Dover, but his usual procedure seems to have been to read everything the author ever wrote (even such famously prolific writers as Dunsany and Chambers), then collect together all the best stories, add in a few more to give an idea of the author's range, and wrap the whole thing up with an introduction that not only outlined the writer's career but told you why he was worth reading. Invariably he made me end up wanting to read more, and usually when I did I found that what he'd excluded didn't match up with what he'd selected.
His GODS, MEN AND GHOSTS  is simply the best anthology of Dunsany's works ever put together* -- better than Lin Carter's, better than Yeats', better than the one Dunsany himself put out near the end of his life (the only rival is Joshi's recent one for Penguin). I've also an admirer of the ones for R. W. Chambers (THE KING IN YELLOW AND OTHER HORROR STORIES), and Ambrose Bierce (GHOST AND HORROR STORIES). The ones for Algernon Blackwood (BEST GHOST STORIES) and Sheridan LeFanu (BEST GHOST STORIES AND MYSTERIES) are equally well done, but I think they show that these authors, while they have a few good tales, don't really have enough to make a collection this large (both in the 350+-pages range, significantly longer than the Bierce, Dunsany, and Chambers collections).
And it was only about a year and a half ago that I got my own copy of Bleiler's CHECKLIST OF FANTASTIC LITERATURE  (pricy, but worth it), in which he listed all the "Fantasy, Weird, and Science Fiction Books Published in the English Language" up to that date. Along with Tolkien's ON FAIRY STORIES essay, published the year before, I think that in retrospect it's one of the starting points of modern fantasy scholarship.
I'm only sorry I cd never think of a reason to write him a fan letter and thank him for all the good work he'd done. Still, I think he must have known how good he was, and how many people out there are grateful for his work.
current reading: MARK TWAIN'S SPEECHES, ed. Albert Bigelow Paine 
*not that it's the same as the one I'd have put together, given the chance -- but then I know Dunsany's work extremely well, having read everything he published that I cd track down, and tracing uncollected and unpublished works whenever possible.
For a concise and heartfelt tribute to Bleiler, see Doug Anderson's post on WORMWOODIANA: