Here are some other places of note on the Internet:
CHRISTIAN HENRIKSSON'S BIBLIOGRAPHY LISTS - Christian has put together complete bibliographies of dozens of great Golden Age writers, and his pages provide great information for those looking to complete their collection of certain authors.
ASH RARE BOOKS - Owner and collector Laurence Worms has a great searchable site of out-of-print and impossible-to-find books; highly recommended if you're looking for that elusive mystery title!
CRIPPEN & LANDRU PUBLISHING - Doug Greene's great publishing company provides an online catalog of their obscure crime and mystery fiction titles and collections.
RUE MORGUE PRESS - Tom and Enid Schantz are reprinting some classic Mitchell titles, and the rest of their catalog spotlights equally collectible Golden Age mystery authors.
HOUSE OF STRATUS PUBLISHING - Another interesting publisher who has brought back into print some long-forgotten crime fiction.
FACSIMILE DUST JACKETS - Mark Terry's great site sells quality dust jacket reproductions from classic Gladys Mitchell and other Golden Age mystery books.
CRIMESQUAD.COM - Chris Simmons's smart contemporary crime fiction site; Miss Mitchell is featured as October 2005's Classic Crime author.
I'll add more links as I go. If you have any suggestions, please e-mail me!