There is this one item in my offline-Chromebook wishlist, long pending. Google Play Books app for offline reading. We do not have such an app yet, but I can share with you today, something closer to that.
If you have downloaded .epub books on your Chromebook, EBook Offline Reader can open them for you, even when you are offline.
There are no options or settings available for this app, for now, you cannot even open an epub file from the app. You have to double click the downloaded file to open it using the app. But it displays the ebook pretty neat.
I will keep looking for a better ebook reader, with some additional options like changing font and font size, etc. If you guys know about anything similar, let me know!