Google Books gets unofficial app for WP7
free books from Google Books, download what you want and read them on the go. Of course, the trouble is right there: this app is limited to free books that are available from Google Books, which are, for the most part, the same free, public domain books that are available through Amazon Kindle. And, the Kindle app for WP7 is an official app direct from Amazon, and so includes the full Kindle store, while GBReader is not an official Google app.
Google has not been quick to support WP7, and has yet only made one official app for the OS - the Google Search app (which has garnered less than stellar reviews.) If you're already invested in Google Books, GBReader could be a good option for you. It costs 99 cents, but does have a free trial.