There were two further volumes in the Elsewhere trilogy, published in 1982 & 1984. Then Faery, a book of adult faery stories, published in 1986, a World Fantasy Award nominee.
In 1989, I began work with Ellen Datlow on The Year's Best Fantasy & Horror series -- and I've been co-editing anthologies primarily in partnership with Ellen in the many years since.
Those early books are dated now and long out of print, but if you'd like to know more about them nonetheless you'll find an article on the genesis of Elsewhere by John O'Neill in Black Gate magazine, and information on Faery in the Fairy Books section of this site.
The drawing above is by Anne Anderson (1874-1952).