Microsoft has released an update to Microsoft Reader to address activation problems that some Phone Edition users experienced. The new version also makes it easier to use storage cards, adds pan and zoom capabilities for embedded graphics and landscape and portrait screen orientations are now supported (on WM 2003 and above)
Note that this version of MS Reader is not compatible with Windows Mobile-based Smartphone devices but supports Pocket PC 2002, WM 2003, WM 2003 SE and WM 5.0 (inclyding Phone Editions).
Personally, I’ve had enough issues with Microsoft’s DRM solution that ebooks in MS Reader format are no longer an option. Sorry Microsoft!