I love a bit of retro style, and these vintage library posters are fantastic! I think they could still be used today :)

RETRO POSTER - The Book Hunter

RETRO POSTER - Books are Arranged ...

RETRO POSTER - When You Want Facts

And for the fiction buffs ...

RETRO POSTER - There's Romance in Books

There's a whole Flickr album of these posters, collected by a library technician who goes by the username Enokson.