In Preview mode, this is where you can view your book in spreads. The flipbook view aims to give you a better idea how your book would look like when printed.
Normally, it should look like this:
However, if your Preview Mode looks similar to this:
You would have to fix your browser zoom settings.
It is highly possible that your view settings are set more than 100%. Kindly set the zoom settings back to 100% to view the flipbook preview properly.
To zoom out, you can visit the Settings panel of your browser or press CTRL and - on your keyboard multiple times until the view looks normal.