www.flakturme.com/index.html this site is under construction but 5 years ago it was top rated with many fotos during the war and after. They are revmaping the site. www.geocities.com/isanders_2000/ww2.htm www.geocities.com/lupinpooter/berlin.htm
another thought and it covers more of the air raid towers/shelters but go here: www.lostplaces.de go through the site slowly and there is some tremendous materials here.
If you can get hold of it, an extremely good pictorial reference - with many photos of the Zoo Tower - is Michael Foedrowitz' 'The Flak Towers In Berlin, Hamburg and Vienna, 1940- 1980' ( Schiffer, 1998 ).