On oz_mystery_readers on Yahoo we recently read Fergus Hume's The Mystery of a Hansom Cab for discussion.
(
http://www.austcrimefiction.org/index.php/Hume%2C_Fergus) if you want some details.
Originally published in 1886 it holds up really really well given the age - one participant in the discussion commented that it's a melodrama (but a good one) and I was struck again by the problems associated with reading books from previous eras. And this from somebody who has read 'em all :)
Personally I loved Fergus Hume's book and I'm really looking forward to getting time to read Madame Midas not just because it's set in Ballarat - which is the closest town to where I grew up. Another firm favourite (although not as old) is the Sjowall / Wahloo series from Sweden - fabulous books which I really enjoy.
What are the "aged" gems that you really enjoy and why?