Via the bbc it seems that the British Library is All books are public domain and often dating back from the nineteenth century. The British library is working in conjunction with Microsoft on the project by utilising its Microsoft's Live Search Books.
The article says:-
Digitised publications will be accessible in two ways -initially through Microsoft's Live Search Books and then via the Library's website.
The books will be fully text searchable, meaning users will be able to look for keywords within a publication, making research easier and enhancing interaction with the material.