Marginal Technology Mashup
Microsoft has released a new version of Microsoft Reader for UMPC devices.
I bought some eBooks for Microsoft Reader back when it came out (almost 8 years ago now), and after paying a significant premium for the books and then coming to realize that I couldn't give or lend the books to anyone, that was the end of that.
Adobe is taking a shot at this market with Digital Editions after also having the Adobe eBook Reader for a number of years, but that technology is now integrated with Acrobat.
I don't know much about it, but one feature that seems like a real differentiator in Digital Editions is support for "borrowing" books from a public library. Also, because it's Flash based, there's no client to download and install. The Flash client installs itself when you read your first book.