Best £12 I've ever spent on biking:
[url= http://www.amazon.co.uk/Mastering-Mountain-Skills-Brian-Lopes/dp/0736083715/ref=sr_1_1?ie=UTF8&s=books&qid=1281605458&sr=8-1 ]Where I bought it, other retailers are available...[/url]
I should add the caveat that I'm a relative newbie and do not have mad skillz, so I'm very much starting at the start. I expected it to be full of useful information but I didn't expect it to be so well written and entertaining (admittedly I am easily amused).
If you liked that, you'll also like this [url= http://www.amazon.co.uk/Fundamentals-DVD/dp/B0006OQYZK ]Fundamentals DVD[/url]...
Good book. The general sense that you do everything firmly, no half measures, is particularly good - you get grip by pressing downwards. Seems obvious, but when you contrast that with the common tendency to tip-toe through the slippery stuff you see that it is good advice.
Also quite a funny book in places.