Stripping the DRM is a bit of a legal minefield. You're very likely agreeing to a contract when you buy an ebook, stating that you won't do such things. It may be illegal in law, depending on where you live (here in the UK it was illegal to rip a CD to put it on your ipod, and possibly still is).
I'd guess it quite likely is illegal, in some way. But would anyone ever prosecute you for it, if you've legally purchased the book? Very unlikely, for the same reason nobody here was sued for ripping their own CDs - the likely outcome would be a big change in the law making such things legal, which the publishers wouldn't like
Here's another dilemma though: suppose there's a "not entirely legal" website, where you can download such things with the DRM already removed? (I've no idea if there is or not for books btw!) You buy the book, and want a DRM-free copy. Stripping the DRM yourself is probably illegal. Downloading a pirate copy is also illegal, but saves you quite a bit of messing about. Either way, you bought the book, the publisher, book store and author got their money.
Is downloading the pirate copy any different from stripping the DRM? (Personally, I'd say yes - because you'd be supporting the dodgy website, which is likely making money for whoever runs it via advertising, and the people running these sites are usually not the people you'd be happy to financially support
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What's really troubling about all this is that the publishers seem to be repeating the mistakes the music industry made with MP3. Things would be so much easier if they made the stores DRM free and reasonably priced, and just went after the pirate websites. But once again, they see the opportunity to make lots of money, and will end up with a huge problem in a few years.