How can I look up words in an eBook reader with Google Translate?
There are some ebook reader apps with can enable you to share book passages. And you can use Google Translate to share them... So select the passage and share it using Google Translate ! For example, the Kobo app for Android and Aldiko (free or €2.49) are ebook readers which enable to share book passages. Google Play Books have a built in feature to translate passages too (it must use Google Translate).

Obviously, you can also copy the passages by hand, open the Google Translate app and use it to translate what you have just copied. It is not quite smart or ergonomic but it works.