Sony Reader customers through the Sony digital book store. That brings the total number of books in the Sony store to 600,000, or more than twice the number of eBooks found in Amazon's Kindle store.
The only catch is that these books are all public domain works, most which were written at least 80 years ago. Not exactly the stuff of the New York Times bestseller list. Amazon, by contrast offers a number of new release titles for around $10.
You also don't need a Sony Reader to access the Google library. Any computer or mobile device with an internet connection will do.