I've never understood the library exemption. It says it's OK if "A nonprofit library... gains access to a commercially exploited copyrighted work solely in order to make a good faith determination of whether to acquire a copy of that work for the sole purpose of engaging in conduct permitted under this title". What does that mean?

"conduct permitted under this title" would refer to Title 17, the copyright code. So the library has to be intending to do something that doesn't violate copyright; like, for example, owning books and videos and lending them out. That's legal and it's the business of libraries.

But what's this about breaking the encryption being OK solely to determine whether to acquire a copy? When would a library need to do that? I can't think of any examples. If a library wanted to decide, say, whether to get a certain DVD, would their decision be aided in any way by using DeCSS and ripping a disk? I don't see it.

Are there some other kinds of materials which might have technological protections where the library would in fact benefit from circumventing them, in order to decide whether to acquire the content?

Just off the top of my head, it might be applicable to Ebooks, especially where there's a need for a compatible reader to access the Ebook content. DVD encryption at least follows a single standard. There's many incompatible existing Ebook formats.