I have put some books in .epub format on my Nexus 7 tablet. I really like the smooth interface of Google books app and want to read my epub books within it. But I can't figure out how to import them to my library.
No, you can't. Google Books doesn't allows you read anything which isn't from google play store and no method to do. You just need to use a reader application, such as Aldiko or Cool Reader. Then import to the app for reading.