Today ebook readers can select words in Google eBooks and look up their definitions, translate them or search for them elsewhere in the book from within the Google eBooks Web Reader—without losing your page or even looking away.

The Web Reader works in all modern browsers and lets you read Google eBooks without having to download them. 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. Read more.

See: Google Translate Blog