If you love books and book cover design, prepare to get lost for a long, long time at Publishers’ Bindings Online.
I learned quite a bit browsing through the galleries. For instance the style known as Eastlake, named after Charles Locke Eastlake (1836-1906) who’s work was the precursor to what later developed to be the Arts and Crafts movement. Also the Art Nouveau and Japonisme galleries are rich with inspiration.
One note: the UI is not always intuitive, to search for all the samples of a given period click the “Search the PBO Database for…” link at the bottom of each gallery.