And don't forget to check out the free sample e-books... it's not just PD, but a mix. Reminds me of a free .lit giveaway that was partly responsible for my initial interest in e-books and MS Reader.
For example, they have these that look interesting enough to install the reader just for the purpose of reading them (well, if I get a UMPC or when it's available on PDAs eventually):
* Don't Know Much About History - Everything You Need to Know About American History But Never Learned, by Kennith Davis
* Death and the Senator by Arthur C. Clarke
* The Playground by Ray Bradbury
* Inside the Publishing Revolution, The Adobe Story by Pamela Pfiffner
Seems like reasonable incentive to give it a try.