The Safari Web Inspector is an open source web development tool built into Safari that makes it easy to proof your content on OS X. The Web Inspector can be enabled within Apple Books on OS X 10.9 (or later). During book development, using this tool allows you to more easily modify, debug, and optimize your book.
To enable the inspector from the Terminal, execute:
defaults write com.apple.iBooksX WebKitDeveloperExtras -bool YES
In the EPUB, select any text and right-click (Control-click on Mac) and choose Show Inspector from the shortcut menu.
Note that this will only work on DRM-free books.