Based on a previous discussion Johan noted:
> > must admit I'm not quite certain I even understand the main point of this message, but the sad truth is that not knowing when something will be fully solved one year later is not very uncommon in software development, because you never know exactly how to solve something in beforehand.
That might be true. But it is better to admit failure than leave a message like that indefinitely. Three or six months might be reasonable, but years is not. Aside from the fact that is a bad software development approach to create a link saying "create a book" only to tell the user that they can't create it anyway in this case there are two such links.
That's like creating a sign leading people to a "working toilet" only to have another sign on its door indicating that the toilet is no longer functional. In such a case a person might have a desperate need to relieve themselves, but might find another place to do so. In this case it just makes the site look unprofessional by needlessly wasting people's time.
My suggestion is to simply remove the link from the sidebar, disable the special page, and restore it if the book feature ever returns.