I'm not surprised you can't see any message: WhatsApp messages are encrypted so your eBook Reader could not possibly play one. Also they are specifically communicated to WhatsApp, there's no way your reader could receive it either. So the question is, what is up with your Reader that it's posting this notification, because it's clearly something spurious?
Have you tried simple things, like clearing the eBook Reader's cache (in case some junk in there is confusing it)? That's probably not going to solve it, but won't do any harm. Clearing the reader's data might work, but that might also lose you other data that you want (settings, history, even books if the reader stores them in its own database rather than in shared storage), so I'd leave that as a last resort.
Do you need the Reader to send you notifications of any sort? If not then you could just turn off notifications from the app and forget about this.
None of which answers why the app is posting this in the first place, but I cannot see any reason it should. I wondered whether this was some ad that had got stuck, but apps should not post ads as notifications under any circumstances.
