Just go!
I'm not so sure about books and videos for training. I'm not badmouthing them, I just don't know either way. I was the first person with a mountain bike in my area, so I had to learn on my own. Few things off the of the top my head:
When you ride with others, ride with others who are somewhat better than you. Not neccesarily experts, but still decent riders.
Ride as many different types of trails as you can, but also choose one trail and keep riding it until you have it wired. This way you can hone a certain skillset in a way that you can't on trails that are always new.
If you have trouble with one trail section or type of trail, turn around and ride it ten times. I was an expert class racer and said, "I have trouble with that section." My riding friends, said, "Well, let's turn around and practice it." It was so simple, but I didn't do that...D'uh!
Think about what you are doing. Some might say you have to feel the trail, yes, of course you do. That's come from experience but if a tricky section is coming up, think what you did before that worked (or didn't work). Talk out loud to yourself (as long as your alone

. There was a section of trail that had changed when a tree down, I would approach that section and say out loud, 'Around the rock to the left then cut sharply to the right to avoid the end of the log, then..."
Always, always, always look where you
want to go and don't look at where you don't want to go. It's called "object fixation". Snowmobilers will hit a tree in the middle of an empty field, why? Because they were looking at the tree. I'm a hang glider pilot, but was out of it for a few years. This summer I started it up again and my instructor would say to me before takeoff, "Why are you looking at the house if you want to go towards the tree?" He would always look where I was looking before I took off. After the lessons, I would go out on the trails, on which I have years of experience, but still, when I would focus on the tree off the side of the trail, I would veer towards it. This is something that we should all focus on (or not focus on...depends on what the object is

.
Keep safety in mind and know your limits and that of your equipment. In many sports, mtb'ing and hang gliding included, the person most at risk is the one who is starting to approach the intermediate level. They are developing skills and losing their caution, but not neccesarily at the inverse level.
Feel the trail!
Enjoy....
Penguin