By then the sound had finally penetrated my sleep and I thought Is something wrong with the toilet again?
Gaaah, I thought. I'll need to check this out. So I got up and headed towards the bathroom.
And stepped into a few millimeters of water pooling on the floor. Great! It apparently had just started happening so the damage to the books on the floor of the bathroom wasn't much, if any. And the rug had absorbed most of the water anyway. The toilet looked okay, so I did an experimental flush.
And watched in horror as the water filled right up to the brim before going all the way down. Very odd behavior. Then I noticed the shower stall was filled with water.
That's not a good sign.
I retrieved the plunger and started to plunge the drain in the shower stall. It didn't seem to be doing anything. Plunge plunge plunge, watch watch watch plunge plunge plunge watch watch watch. Then suddenly the water started draining out, but I'm still not convinced my plunging had anything to do with it.
I took an experimental flush of the toilet. It worked fine.
Now what was that all about?