I think there are more pianists than 50 who did the complete sonatas, no? Now when I'm doing it myself it feels like I notice people doing it left and right...but that might be me going a little nuts

I guess performing them within a fairly short time (4-8 weeks) is not so common. I just noticed that one of my old teachers will do them in Stockholm, and I know for a fact there are many, many of them he hasn't played yet, Hammerklavier being one of them. I can say right now: that will not be a fun experience for him. The last year before the cycle, one should not be learning notes but already having played them all at least once. And if you keep learning them, 2-3 a year, all of a sudden you realize whoa! I have played 25 sonatas!
Ha, I was thinking to myself after writing about recordings "damn, I forgot Annie Fischer!" Her op. 110 is one of my absolute favorites.
Birba, would you share some more of your experience at the courses with Kempff? If you do, I would love to read it.
I will check out the Taub book. Curiously, one cannot buy it on Amazon, it says something is wrong with it?
The studies and books I have found most interesting so far are books that handle music and history before Beethoven: to get to know J.P.E. Bach's music, the sensitive style, galant style, the mixture of them, Rousseau's influence on politics and arts and more. All those things put Beethoven's (especially his early) music in new context to me (sounds pretentious, I know, but it's great stuff!).