The biggest difference between mountain biking and road biking is that the off road terrain is quite irregular with considerable variation in rider energy output from minute to minute as one covers repeated up and downhill pitches rather than the more predictable steady grades and level stretches found on most road rides. For that reason, the mountain biker will tend to emphasize interval training (the comments on training mileage are relevant, but with at least one and possibly two interval days per week). Substituting a hill for your "interval" instead of picking up the pace on the flats offers a more realistic simulation of what will happen off road and will help train your legs as well as your cardiovascular system. Using a heart rate monitor to avoid overtraining from underestimating true levels of exertion is also helpful. And if you are trying to determine your principles of nutrition are the same as for road biking, and specific dietary recommendations are outlined in the BASIC TRAINING RIDE, INTERVALS, and COMPETITIVE EVENT sections of the "Nutrition plans for 6 common types of rides".
The remaining sections (myths, training tips and aids, etc.) are equally relevant for mountain and road biking. The only section unique to mountain biking is on infections due to the off trail and forested terrain.