> That is bad for your knees.

[Citation needed]

I wouldn't recommend speeding up on the downhills for beginning runners. But if you're well conditioned, running slightly faster on the downhills in a relaxed way won't hurt your knees. It all depends on the slope of course. On very steep hills, it simply isn't be possible to run fast downhill.

> I highly recommend the book Born to Run by Cristopher McDougall.

It's an entertaining read, but that's about it. The science is to be taken with a large grain of salt. (I've run more than 50000 km since reading it.)