British author and editor Peter Haining, who died in 2007, put together dozens of anthologies that collected classic and obscure horror, occult, Gothic, and science fiction over several decades, as well as nonfiction works on our beloved genre. Virtually all of these books were published, for whatever reason, only in the UK. Some - if not most - of the US paperbacks:
One of Haining's nonfiction titles that made it 'cross the pond is Terror! A History of Horror Illustrations from the Pulp Magazines. Fortunately I found a copy when I was in high school. It really and truly fed my hungering heart of horror, and it's been an invaluable resource on my bookshelves for decades now! Which is just the way he would've wanted it, I'm sure.
Read more about Peter here, and see lots more of his UK covers here.