Francois gave us a very interesting introduction into the history of this quite special library, its unusual classification system and their digitisation projects. http://warburg.sas.ac.uk/mnemosyne/entrance.htm shows the organisation of the library by subject, which reflects the main interests of the library's founder Aby Warburg. The chart is linked to a list of subjects which then is linked to the catalogue. This certainly is a great additional way of making the library's rich collection accessible.
Their digitisation projects started in 02/03 to increase accessability of rare and early printed books. The main problem they faced was that digitisation is quite time consuming so they have now mostly outsourced the actual action of taking pictures. Also they now work together with artstor, the picture database equivalent to jstor (great to browse: http://www.artstor.org/index.shtml). They have so far 530 books online, accessible through the catalogue. And Francois clearly pointed out: Through digitisation you reach a lot more people.