How do I change sharing permissions and who can I share my ePhotobook with?

You can go to Share it under My Account, then find the book you wish to share. Click on Share, then choose between Restricted (where you will need to key in the email of the person you wish to share with) or Public (a link will be available where you can copy and paste to send to the person you wish to share with).  Do note though that when the person you share with tries to open the preview, they would need to log in through their Photobook account or via Google or Facebook to be able to access it.