20th century science fiction, fantasy and horror syllabus! Please please!

I will admit I only read some Wellman when I was about 17 and forked over two months allowance for a lovely hardcover of selected stories, and though they made a strong impression they are probably a. just the idea of stories I liked and b. were selected to be a best-case intro to his work.

(And I can't believe I mispelled "particular". How embarrassing.)