According to Book Help, I should be able to Collect articles if I "hover over a link for a few seconds". This does not work for me. I have "Navigation popups" enabled, though disabling it makes no difference. I'm accessing Wikipedia (Vector theme) with MediaWiki 1.24wmf7 (b8870ae) with Collection v. 1.7.0.
Perhaps Wikipedia articles that are part of a book, could have their links styled differently?