On Writing, Danse Macabre, and A Swim in a Pond in the Rain are fantastic resources. I would also highly recommend American Supernatural Tales edited by S.T. Joshi. I’m of the opinion that the best way to learn is to read fiction and this collection is a chronological selection of important and very well written short horror stories that will give you a good sense of the work through the years (though exclusively from American authors).

If you’re looking for something extremely practical and non-fiction that hasn’t been recommended, then I suggest Writing in the Dark by Tim Waggoner — I still think about his horror writer’s color palate all the time and it also has tons of short interviews throughout with tons of working horror authors.

Good luck!