Hello there!
Sorry for the long silence... I have been going crazy with my dayjob, not much time left for coding lately =(
Glad to know you managed to work it out!
Just a reference if anyone else stumbles upon your question: the "Replace" command will remove all currently visible text from the book, and make visible only the text index you point out on the command. If the index you provide has no text, then the book will be shown empty in the text window. However, if what you want is to add another chunk of text to be shown (instead of replacing all of it), you should use the "LibraryData TextUpdate key Add index" instead!
Just keep in mind that the provided index must have a text assigned for it in your datasource, be it an external JSON file or the Plugin Manager. If the index does not exist or has no text, the plugin will simply assume there is nothing to display.