[murraypaul]

As an alternative answer:
Your preferred future-proof format might be LIT.
My would you want a Microsoft format for your Sony reader?
Because
a) It is trivial to strip the DRM out
b) Once you've done that you can unpack the LIT file and get at the actual HTML file inside it.
Then you have a standard non-restricted HTML file that can be read by or converted to any device you want.
You can also make any changes to the layout, font faces and sizes, etc... that you want.
You actually have the book text, rather than just the ability to use certain applications to view the book.