There must be some book with actually banned content in it, right? Especially copyright violations. They could include a PDF of some Linux source code but with MIT license.

Edit: per the other comment's Wikipedia link, the unredacted Operation Dark Heart seems banned in the US because it included classified info

There is a book called "60 Years Later: Coming Through the Rye" which is (was?) banned in the United States: https://en.wikipedia.org/wiki/John_David_California

Although copyright-infringing books may be illegal to redistribute in general, the difficulty in determining what counts as copyright infringement and what counts as fair use means you can't really tell for sure which books are illegal to distribute and which aren't, so I'm not sure that really counts as "banned". 60 Years Later had an actual court order which makes things a little more concrete.