Nah, you're using the plugin commands right. You're using MV's commands, but should work fine too. Also, I've seen this behavior you describe on past versions and it would regardless of how many or which books were available for the player. If it is what I think it is, there may be a combination of parameters for BookMenu causing some kind of scenario I didn't foresee (probably related to background image settings).
Can you send me the plugins.js file from your project (it's inside the js folder)? It stores all your plugin parameter settings. With that I can copy all your BookMenu settings into my test project.
Also, run your project, then press F12 (usually, if it doesn't work press F8) to open the console. If there is any error already there, print it and paste it here for me. If there's nohing on console, run this set of commands and send me the output after accessing the book menu, but before selecting any book:
SceneManager._scene._bgStackSize
SceneManager._scene.children
Then select a book and have the error occur. If any errors pop up in the console, send them to me. Also, run the following commands and send the output as well:
SceneManager._scene._bgStackSize
SceneManager._scene.children
SceneManager._scene._textWindow._bookKey
$dataBooks._books["<BOOK KEY>"].customBgMode
$dataBooks._books["<BOOK KEY>"].customBg
For the last two commands, replace <BOOK KEY> with whatever is returned by the third command (the one ending with _bookKey).
I hope that helps me find out what's going on. I tried all the parameter values I thought could cause this, but it's all working on my test project. There may be something else that I'm missing.