https://www.nbcmiami.com/news/local/roughly-300-books-were-r...
Browsing the list of in rem cases in the US turns up quite a few cases where the US actually seized several pornographic books such as Lady Chatterley's Lover, which is perhaps a little more in line with what you were thinking about.
The last guy's alluding to right wing books though. I remember Axios reported that the Turner Diaries and Camp of the Saints are going for hundreds now, because printing has been ended: https://www.axios.com/2021/01/28/racist-novels-skyrocket-in-... But books are now available on Amazon for cheap and pirating is easy, but here are some which libraries don't seem to have stocked and mostly aren't purchasable.
I imagined Holocaust denial etc. are effectively banned/not stocked by most libraries. Googling, I found discussions about My Revolutionary Life by Leon DeGrelle, Into the Cannibal's Pot by Ilana Mercer etc. But things from other movements too like an Ulster loyalist Ian Paisley's Messages from the Prison Cell or Uncomfortable Questions for Comfortable Jews by Rabbi Meir Kahane. Pinochets memoir is also inaccessible.
Then there are books about bombs or e.g. Put em Down, Take em Out by Pentecost about knife fighting ($60/now used). In the UK, a nazi was jailed 13 years for having a pdf the White Resistence Manual: https://www.itv.com/news/calendar/2023-08-31/prison-officer-...
I'll stop there. There are long lists, with about half the entries seeming plausibly banned.
It's the definition of an imaginary threat.
The books that are actually banned, the ones that were forced out of print, not available in public libraries and cost hundreds or thousands for a used copy? They are right-leaning politically.