Google's eBook Reader for the Web added a contextual menu that lets you define, translate and search for a selected word or text. "To select text in a Google eBook within the Web Reader, double-click or highlight it with your mouse and a pop-up menu opens with the following options: Define, Translate, Search Book, Search Google and Search Wikipedia," explains the Google Books blog.
Definitions are obtained from Google Dictionary, while translation is powered by Google Translate. If you click "Search Book", Google shows a list of all the instances in which the selected text appears in the book.
These features aren't available yet in the Google Books apps for Android and iPhone.