Genghis, I have to confess I don't know of any official document that says the "normal" circuit height is 1,000 ft - I guess that one reason for originally picking 1,000 ft is that when flying downwind at 1,000 ft the hundreds pointer on the altimeter is at the twelve o'clock position on the altimeter which makes it a bit easier for the inexperienced/student pilot to read and/or notice any deviations.
I suppose some ancient practices which have been established since the days when Pontius was a pilot aren't written down in referenced statute almost like the fact that (in the UK) we drive on the left hand side of the road - if you happen to find a reference please let me know!