This is great fun. Its the British Library Online Gallery 'Turning the Pages' project. You will need to install the Shockwave plug-in if you don't already have it (make sure you unclick the install of Yahoo toolbar - unless you want it) and allow pop-ups.
There's only 15 books at present and they're not all painting related, there's Mozart's musical diary for instance and the First Atlas of Europe but there's also Leonardo's sketches and Flemish Masters in miniature.
You can pass a magnifier over the page and listen to or read accompanying audio/text.
Hopefully there's more on the way.

Link: http://www.bl.uk/onlinegallery/ttp/ttpbooks.html