Yes, you can read your eBook even if you are not connected to the Internet.
Click on the cover image of the eBook that you'd like to read offline to load it onto your computer or device:
A download bar will appear to indicate the loading progress:
Once the download is complete, a notification will appear. The title and reader itself have both been cached to your browser, so you can also read offline if you wish:
We recommend that you bookmark the library reader in your browser to make it easier to access it while offline. If you don't bookmark it, you can also navigate directly to the library reader URL in your browser even while offline.