The books, all published before 1923 and now in the public domain, were digitised by Google as part of its Google Book Search programme. Sony will offer them alongside the 100,000 or so books still under copyright that it sells through its eBook store.
It's already possible to download public domain books from Google Book Search as PDF files and copy them onto a flash memory card for use in e-readers such as Sony's, but this partnership will simplify the process for users by integrating it into the eBook Library Software for PCs that ships with the Sony Reader.
Sony Readerowners can get their hands on hundreds of free titles
The service won't give Reader owners free access to the much greater number of books still in copyright, many of which Google Book Search has already digitised, however.
To access the public domain books, owners of Sony's PRS-505 or PRS-700 readers will need to install the PC software and create an account on the eBook Store if they don't already have one. Owners of the older PRS-500 are out of luck, though: the service won't work with that device.
The expanded library won't address one key difference between the Sony Reader and Amazon's Kindle e-readers: the Amazon devices don't need to be tethered to a PC to download and install new books. Instead, the Kindles, which are not currently available in the UK, download books over the air via 3G mobile networks in the US.
However, the network Amazon has chosen for the Kindle is not available outside the US. This leaves an opening for other e-reader manufacturers to provide devices that work with European mobile networks, perhaps linking to other online bookstores.
Google's partnership with Sony is not the first time it has simplified access to Google Book Search for mobile devices. In February, it opened up the service to the Apple iPhone and to phones based on the Android software platform it backs.
The phone interface offers access to more public domain books than Sony's eBook Store: up to 1.5 million in the US and 500,000 elsewhere, according to Google. Amazon followed suit in March, with the launch of an iPhone reader for ebooks sold through its Kindle service.