Note: I am adding these comments from a previous user post for the benefit of everyone.
If the preview does not complete successfully then it means that one or more stories in the eBook have suspect HTML coding and cannot be properly decoded.
In such case you need to remove the offending story(s) in order to complete the preview. You can do this in the "customize" section where you can also add in (and edit) the other stories later.
If the progress bar is stuck on a particular story and not moving further then you can only recover from this state as follows:
The solution is to go directly to this URL to customize the eBook that is being created:
On that page you can remove stories that are causing problems.