I’ve collected a list of useful websites dedicated to the Berlin U-Bahn:
H & Co.’s Youtube Channel for Berlin U-Bahn videos
http//www.bvg.de (Official Website)
https://en.wikipedia.org/wiki/Berlin_U-Bahn (Wikipedia entry)
http://www.berliner-untergrundbahn.de (Extensive information on lines and trains)
http://www.urbanrail.net/eu/de/b/berlin.htm (Photos, books and general information)