Unfortunately, I'm still unable to replicate the issue ='(
I created a clean project and copied your files over the default ones, then added a single event that adds some books to the library and run the project. But it works fine, no errors, even if I load all books at once.
The behavior you describe would happen when the plugin tries to handle dynamic background images (even if there is none configured). The ToastManager plugin adds a new layer to the scene stack, which caused issues in the past. I thought I had reinserted the same issue on version 1.6.1, but that does not seem to be the case, as the steps to replicate it then are not triggering it now.
The files you sent me... are they from your current project or a test project?
If you can setup a test project with a single map and only default resources where we can easily replicate the issue it would be really helpful. You don't need to send me the whole project, just the data and js folder, and the book json file if you use one.