My favorite library, the Staats- und Universitätsbibliothek Hamburg, owns a lot of interesting old books - some in desperate need of repair. To raise money for restoration they present books on their website. Now that's a great idea - they can find somebody to finance the restoration and I can take a look at treasures they normally hide in the archives :-)
So take a look at Buchpatenschaften
My favorite is Ephraem Syrus: Sermones from the 15th century, a small volume with wooden boards and a simple leather cover.
This charming little books is the inspiration for a Rechenbuch I'm working at at the moment.
Pictures of my version will follow, promised!