Well, this is a perfect way to start a conversation!
Your SIL's son is asking these questions, and I think you're saying that your MIL is the one you want to give the book to. So in the context of helping your nephew, you could mention that you've found a system that is working for you, works for others, and actually talks *against* the need to build credit. That it's about not using credit except maybe for a mortgage. And so on.
Honestly, I built my credit starting at 17, with 3 cards I got in college, and by 27 I had destroyed it all b/c it's funny, they will give you cost of living loans in graduate school, but all of that ends once you're graduated. The damage I was doing while in school took a bit over a year of being OUT of school to take effect, and by the end of that I had NO cards and destroyed credit. So...yeah...building credit (unless he's going to go buy a house NOW) at 18...is difficult if you don't have really really solid knowledge.
So start the talks about your nephew. If there's interest, the SIL will tell you.
Since the SIl is calling, you should just talk to her. Don't send them books unless they ask about them, and they might ask about them b/c you could give her basic info on the phone during those calls.
My friend gives books to her friends, once she is done with them. One book she gave me was Rich Dad Poor Dad. I was interested in it, until it was given to me. Iv'e never opened it. But I found out about Ramsey through the Dis, and just like rewards websites, first I thought it was nonsense, but now I'm coming around very nicely, and seeking out the info on my own.