May want to check this out.
Looks like that you can upload all your books no matter when you get them to your skydrive and view them on the phone.
There are also alot of sites on internet then you can get books from then and use them on the phone rather then relaying on one particular service.
I don't think they have google books for it yet though.
Check with your public library, too. Here in the Phoenix metro area we have access to 14-day digital book checkouts through a service called OverDrive.
OverDrive provides a Windows Phone app that works fine. A lot of their books are also available in Kindle format in which case they will work with the Amazon Kindle app on your phone. You could try both apps and decide which is the better reading experience for you.
I prefer to read books on my Kindle Touch, but sometimes the library checkouts are only available in EPUB format in which case I have to use the "OverDrive Media Console" on my Radar 4G and that works great (although the screen is so much smaller).