i searched the internet for hours, but didn't find a satisfying answer to what's the difference between the first and second version of HTDP (How to design programs)?

Why should I start with the second version, instead of the first one?
As the second edition is not completed yet, I fear that I might get to the point where I have to switch back to the first edition.

The problem is:

1. To which part/chapter of the first edition?
2. Can I switch back and forth at all? Isn't the second edition totally different structured? Or are the individual chapters between those two version compatible?

I hope, someone can help me.