If you create a book compilation of "Wikipedia's greatest hits", then you're stuck releasing the book as ShareAlike or negotiating with the copyright holder(s).
If you write a book about a particular topic and you use excerpts, quotes, or ideas from Wikipedia (and properly cite them), that's fair use. I'm sure there's some sort of ceiling on that (IANAL) which prevents you from copy+paste-ing broad swaths of text into your work and "citing" it.
The key is the citation, which is where the author screwed up. If you're using text/ideas from a particular source and not citing that source, then you're not covered under fair use and are obliged to follow whatever licensing system the original author wishes.