To get started running downhill more effectively check out An Introduction to Downhill Trail Running Techniques and Going Down! A Guide to Descending on Trail …
To get even faster, read Controlled Chaos: Learn to Be an Elite Hill Runner.
Running a race with tons of descent can leave you quads shot during the race and aching incredibly two days later. Read up on Delayed Onset Muscle Soreness and Recovery. Avoiding Quadraphenia, and Protect and Preserve: Quadriceps Training and Race Preparation.