My brother is in the Marines.  I'd like to get him a PDA for reading
ebooks while overseas.  Ideally, this would be something that doesn't
drain batteries very quickly.  (If the weight of batteries required to
read an ebook is more than the weight of a hard copy, that wouldn't be
helpful.)  Fancy color display is unnecessary, but I think it will
need a lot of memory.  I'd prefer for it to hold ~15 books.  No need
to do any research if you already have a pretty good idea what the
answer is.  Answer should include the following information:

1. Whether this is even possible.
2. Approximate amount of memory required per ebook.
3. Approximate amount of memory required by operating system and other
unavoidable software.
4. Approximate lifetime of a PDA on a set of batteries.  (Assume it's
being used for reading, not in some kind of standby mode.)
5. Any info or key words I could use to research removable memory
options.  (Maybe I could put ebooks on something like the little cards
that digital cameras use?  But this could get expensive if I'd need a
lot of cards.  I can research this myself if I know what words to look for.)
6. If you happen to know of any models that would be appropriate for
this purpose, I'd appreciate any suggestions.