[moefh]

As far as I understand, the "anti-circumvention" provisions of the DMCA don't make exceptions for fair use, so it's illegal to circumvent copyright protection even if a fair use defense would mean you're not infringing the copyright.

From Wikipedia[1] ("1201" here refers to the DMCA anti-circumvention provisions):

  Although section 1201(c) of the title stated that the section does not change the underlying substantive copyright infringement rights, remedies, or defenses, it did not make those defenses available in circumvention actions. The section does not include a fair use exemption from criminality nor a scienter requirement, so criminal liability could attach to even unintended circumvention for legitimate purposes.

The DMCA does include exemptions that allow you to circumvent copyright protection in some circumstances, but these are pre-defined by the government every 3 years. I don't think "backing up e-books that you own" is currently exempted, the only thing I can find in that Wikipedia article that could maybe fit is this:

  Literary works, distributed electronically, that are protected by technological measures that either prevent the enabling of read-aloud functionality or interfere with screen readers or other applications or assistive technologies, or for research purposes at educational institutions;

In other words: if you have an e-book that doesn't provide accessibility functions, you can crack it in order to be able to read it.

[1] https://en.wikipedia.org/wiki/Digital_Millennium_Copyright_A...