It's a shame that The Underground Conspiracy by Catherine Storr was first published in 1987 as it meant it didn't get the sort of publicity it would get now, of a children's book crossing over into adult territory, and a great read for adults and kids alike.
The story is about Jass, a teenager who loves to travel round on the Tube, just for fun. However, her underground activities land her into quite serious and frightening trouble, when she is blackmailed into carrying packages around on the system she knows so well. It's basically drug running on the London Underground which is a remarkably sophisticated premise for a children's book, and I really urge you to read it. For tube geeks you'll see how her extreme knowledge of the London Underground system comes into play.
It's also a great gift for friends and family who may like "The Curious Incident of the Dog in the Night Time" (which incidentally has a great London Underground section which describes an autistic teenager's very first Tube journey - he loses his pet rat on the tracks, almost electrocutes himself rescuing it, gets on the Tube and notices every single sign, ad, person and piece of upholstery). Like The Curious Incident, The Underground Conspiracy for a while has that same naiviety of a teenager mixed up in an adult mystery and not quite knowing how to handle it.