While the plugin works, I ran across an extremely stupid quirk that made me lose hours to debugging.
I uploaded a document before switching my WordPress site over to use HTTPS. When I went to view it, instead of showing the flipbook, it return an extremely vague "Cannot access file!" error.
This was because it was trying to get the file over HTTP, causing the browser to block the request because of mixed content. For some reason, when you upload and select a file using the plugin, despite it using WordPress' media UI, it saves an absolute URL, including the scheme / protocol. Instead, it should refer to the PDF in WordPress natively, or at least use a more relative address.
The fix is simple. Go to the uploaded book and manually change the URL to https. It would've been nice if there was better documentation for this. Or if it just worked properly in the first place...