I first encountered the Pan Book of Horror Stories at my public library when I was in high school. They were some of the later editions, all of which featured a story by Stephen King. I read all of them that the library had.
I didn't encounter them again until after I had turned thirty and came across a cluster of them in a used book store for about a dollar apiece. I still only have less than a third of the total thirty volumes that were published, all edited by Herbert van Thal, and then Clarence Paget for the final five volumes, and containing a mix of short stories of varied quality.
I've never actively sought out any of the volumes I'm missing, but may do that some day. In the meantime, here are the covers for the first twelve volumes, including as many variant covers as I could find. You can view the contents of each at the official Pan Book of Horror Stories website. A new reprint of volume one was published at this time last year. It's already gone out of print, but can still be found inexpensively.
Come back tomorrow for the covers to volumes 13-30.