Do Apple help books, of the MyApp.help format (included in the app bundle), authored for inclusion in OS X apps (from the Help menu) look for *@2x.png images on retina screens?
I don't see any mention of this in the Apple Help Authoring guide or the High Resolution Guidelines.
Help bundles are just collections of HTML pages that are displayed in a web view — so you can use @2x images just like you do on web pages. (Plus, I just tried and you can clearly see that retina content works!) For more details, see this page on Apple's developer site.
