This listing of mail-order study groups at the back is really interesting to me. Growing up with the internet means I never had to use these tools, with such slow feedback, to connect with people over niche interests. But even with a global indexed internet, I feel like I'm yelling to the void sometimes. So I love that somehow at the back of this hand exchanged book, there's a 5 page list of addresses ready to receive your mail.
I'm not sure you could call it a "network" exactly, since it's probably meant to connect you with your closest local community. It makes me wonder what it would be like to participate in a snail mail continent-wide network though! The quality and type of content must be much different than me just spouting my thoughts to the open web!