One of the most prolific and well-known of the original pulp masters,
A. Merritt had his fiction published for decades by Avon. These are the 1970s reprints, featuring some pretty stellar paperback cover art! As his Wikipedia entry states, "Merritt's stories typically revolve around conventional pulp magazine themes: lost civilizations, hideous monsters, etc." Oh,
etc.: love it! Also: "His heroes are gallant Irishmen or Scandinavians, his villains treacherous Germans or Russians, and his heroines often virginal, mysterious and scantily clad." Oh, scantily clad: love it! Honestly I don't think I've read anything by him, but these would look pretty great on my shelves...
