Ash-Tree Press was founded in 1994 by Christopher and Barbara Roden, as a way to return many excellent but neglected ghost story collections to print. As Ash-Tree maintains their own web site, here are links to various areas on that site.
Links to the Ash-Tree Press web site:
Ash-Tree Press History
Ash-Tree Press titles in print
Ash-Tree Press titles out of print
From the Tartarus Press web site:
"The Tartarus Press was founded in 1990 by Ray Russell, and was intended primarily to reprint works by and about Arthur Machen. However, over the last eight years Tartarus Press has diversified into a wider field, publishing works by a number of other writers ranging from Edward Heron-Allen and Alain-Fournier through to contemporary authors such as Peter Cannon and Mark Valentine."
Tartarus Press Web Site
Midnight House is: "Dedicated to the preservation of classic horror and weird fiction in handsome, inexpensive editions". So far they have published works by Edward Lucas White, Jean Ray, Dick Donovan, W.C. Morrow and others.
Midnight House Web Site
Sarob Press is a new imprint doing: "quality limited numbered editions of classic and modern supernatural novellas, novels and short story collections". Their first release was CARMILLA by J. Sheridan Le Fanu, followed by collections of Clive Ward, Bill Read, S. Baring-Gould, and a series of titles by overlooked "Mistresses of the Macabre".
Sarob Press Web Site
Additional publisher information can be found in the alt.books.ghost-fiction FAQ.