If you've never browsed the New York Public Library's Digital Gallery, take a few hours out of your day and browse. It's like looking at a iceberg, you keep finding more below the surface. This collection I found interesting in comparing book design from then to now. For the most part the designs are limited to one or two typefaces and 2 colors at most. In contrast to today's multi-layed photoshop monstrosities. It's said to not judge a book by it's cover, but perhaps it's why I don't read novels anymore.
There's many other collections to browse as well.