District Dave is a tube driver with an extensive web site.
Going Underground has a blog which is great, and much more up-to-date. Annie Mole is responsible for these, you may have seen her on the BBC's tube night (great links here!), in which we saw the caring folk of the Lost Property Office. If you did, you'll be pleased to know that the family who left the ashes of a relative on a train have been tracked down: Underground Urnie found.
Book: Roberts, M.J. (2005). Underground Maps after Beck. Harrow Weald: Capital Transport Publishing.
London Underground Railway Society.
Tubeprune is not updated any more, but has lots of information on the London Underground railway system.