I don't know how many of you are already aware this exists, but the British Library has a ton of amazing scans of 19th century text & illustrations on Flickr. They're fascinating to browse through but also incredibly useful since by now they're all well into the public domain. Some of the illustrations are just begging to be used by amateur poster designers, and there's plenty of treasures for font artists looking for ideas and a HUGE challenge! (I highly doubt anything close to many of these hand-lettered titles exists in digital typeface form)
So far one of my absolute favorites is this old book cover, I would kill for a font like the one used in the main title!
And the old comic / story book covers have a wealth of friendlier, informal type designs:
Any long-forgotten antique custom-letters or typefaces that make you drool?