May I know the version of Lightroom?
Have you saved your book in Lightroom?
I'm using Lightroom CC. The book is saved. I've been working on it on and off for two months. This just started to happen.
I am having the same problem!!! How were you able to resolve it?
Please let us know the Lightroom and Operating system version your are using.
Also, try and update the Lightroom to the latest release 2015.5.1