Are you referring to WordPress standard categories, minisite taxonomies, or minisites themselves? If books is your minisite and you have the top 10 widget in a sidebar and you're viewing from a minisite page, it will automatically only show you only articles from within the books minisite. However you can't limit it by category or taxonomy without modifying the code in the functions/widget-top-ten.php file
No I don't use the minisite. I thought I did'nt need one, because I only use the like function. But if I understand you right, I should make all my categegories a ministite to make it work.
The only way to get "category-specific" top 10 widget is to use minisites, as it stands now you can't limit it to certain WordPress categories without some theme modification.