If you're starting with a support group, you'll need GUIDEBOOK I. If finances are limited, just do this.
I recommend getting all 3 books up front if you can afford it. Benefits: There's things in GUIDEBOOK 2 that you may want/need to know right away. You may want to blaze through the books right away if you're hungry for information, then you go through them again in the group and work on things.
The Biography is a great place to start reading. I suggest only reading the second half (Susan's Unintended Journey) now, and save the first half for later.