I’ve just submitted version 5.2 to the appstore. Pending approval process, it may take a day or two for the new version to be available for download.
This version supports using Books app instead of iTunes. Of course, it is possible to continue using iTunes if you have not upgraded to Catalina yet. However, the build options are slightly different now. I recommend reading the How to Use section.
You may also run into the following issue when adding books to Books app.
There is an issue in Books app which may cause confusion. If a book by the same title already exist in books, the new book will NOT overwrite the previous version! Please delete the book by same title from the books app before building one with same title. This may happen as you are experimenting with building the same book many times.
There was a version 5.1 if you are wondering. That version had a nasty bug that had to be dealt with immediately. We don’t speak of version 5.1 anymore.