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Have a look at DRM in music. Then extrapolate.
NONE of the drm-systems available on the market today will be available in 20 years. Not a single one. Perhaps there will be converters or you will be able to download your book bought today in the new .mobi3-format. Probably is not likely.
You would have to hope that
a) the company keeps their user-, account- and bookshelf-information for 20-years - which they dont want to do for a variety of reasons
b) the company allows you to download the book in a new drm-format - which they dont need to do
c) the company doesnt loose its contract with the given publisher - thus not being able to let you download the book in the new format, even if they want
d) the company still exists
etc etc
And dont get your hopes up with PDAs or PCs. No one knows what kind of system will be used in 20 years. Just one thing is sure: It wont be anything like todays systems.
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