From the website:
Early Music Online is a pilot project in which 300 of the world’s earliest surviving volumes of printed music, held in the British Library, have been digitised and made freely available online.
You can explore the digitised content via the British Library Catalogue. Included are full details of each digitised book, with an inventory of the contents of each, searchable by composer name, title of composition, date and subject, and with links to the digitised content. (Click ‘I want this’ in the Library catalogue to access the digitised music.)
(The British Library link takes you directly to the collection and not to the British Library homepage.)
There are a number of uses which suggest themselves for this data.