If you're that serious about keeping the contents of your e-reader reasonably private in terms of material you think may be subject to arbitrary and unreasonable confiscation which would include seizing your entire reader, you really have two choices:
1) Delete potentially offending material off your reader before entry, but keep an online backup that you can use to reload it when you've left the country.
2) Hide in plain sight: don't bother encrypting your potentially problematic books, just alter the metadata to give it a less tell-tale title. Most searchers probably won't bother looking through each and every book on your reader to see what the contents are.
And if you're worried about that, you can always do a sort of hollowed-out-book thing where you take the text of one e-book and sandwich it in between the text of another, blander book.
It's easy enough with Amazon's Mobi format to have an entire concealed sub-book complete with navigable chapter marks stuck within a single chapter of another book, which won't show at all during cursory examination of the wrapper book, due to the way it renders anything but 1st-level NCX navigation functional but virtually invisible, as far as the Kindle user interface goes.
I will recommend choice 1) as the optimal solution unless one is truly determined to smuggle one's e-book collection of
popular banned books around the world in order to prove one could do so (and maybe have photographs of said titles loaded up on the e-reader in recognizable landmark locales the way some people do those "garden gnomes around the world" photos).
Anyway, for truly anonymous use of any e-reader, you'd pretty much have to have the sort of reader which doesn't require registration to unlock its more useful functions (or be willing to skip use of said functions), and load up books that are either DRM-free to begin with, or can be made so in order to not require registration, and thus, identifying personal information, to use them.
As for international use, leaving aside geo-restrictions, the DRM does kind of mess with you in that regard. B&N openly blocks e-book purchases from non-US/Canadian IPs unless one has a US military e-mail address, and I can imagine handling ADE would be pretty difficult if one doesn't carry around one's own laptop for downloading the files to before transferring them to the reader and has to rely on random computers available via internet cafés and so forth.
Assuming one wants to get books by major authors/publishers (nearly all DRM-ed), one would pretty much have to register one's reader in order to be able to obtain the books and load them wirelessly without use of one's own computer while overseas.
Of course, DRM-free books don't come with these hassles, but your reading selection kind of narrows a lot unless you really like a) computer technical how-tos, b) science fiction and fantasy, c) erotic romance, because those are the main categories which are available DRM-free outside of amateur self-publication.