The titles will span the period 1700-1870 and thus will include many well known titles but it is likely the unique materials which will be made available digitally will be most interesting. As with similar deals with other "Google Libraries", the deal is not a financial one and Google will cover all the costs of the digitization and provide a copy to the BL and keep one for itself.
More from their press release:
This project will digitise a huge range of printed books, pamphlets and periodicals dated 1700 to 1870, the period that saw the French and Industrial Revolutions, The Battle of Trafalgar and the Crimean War, the invention of rail travel and of the telegraph, the beginning of UK income tax, and the end of slavery. It will include material in a variety of major European languages, and will focus on books that are not yet freely available in digital form online.
The first works to be digitised will range from feminist pamphlets about Queen Marie-Antoinette (1791), to the invention of the first combustion engine-driven submarine (1858), and an account of a stuffed Hippopotamus owned by the Prince of Orange (1775).
Once digitised, these unique items will be available for full text search, download and reading through Google Books, as well as being searchable through the Library’s website and stored in perpetuity within the Library’s digital archive.
Researchers, students and other users of the Library will be able to view historical items from anywhere in the world as well as copy, share and manipulate text for non-commercial purposes.
Dame Lynne Brindley, Chief Executive of the British Library said: “In the nineteenth century it was an ambition of our predecessors to give everybody access to as much of the world’s information as possible, to ensure that knowledge was not restricted to those who could afford private libraries. The way of doing it then was to buy books from the entire world and to make them available in Reading Rooms.”