It is protected by copyright, like almost everything with a lapse of originality created nowadays.
You can use it under the terms of the license Creative Commons Attribution ShareAlike 4.0, as noted by user6726.
Plus, you could also ask for an additional permission from the author. Some might require some monetary compensation in exchange of giving you an additional license. Others would be fine with just being credited by name, dropping the requisites of linking to the license and their original work. Others won't care at all. I think most people on world building SE would be delighted just of getting a copy of an actual book where their idea was used and they appear credited for it.
While not strictly required by the license, I also recommend you to explicitly thank for their answers and tell how you ended up using it, even if you can only that you indeed had the book on print a long time later (StackExchange will notify of new comments on your answers). It's very nice when people receive such kind of feedback.
(Obviously, in addition of having upvoted the answer when you found it)