You’re welcome. Just a heads up though, so you don’t get too confused: Tobira itself explains the difference later, but it says いけない tends to be used to impose a sense of obligation on the listener, whereas ならない tends to be used to discuss one’s own obligations. Sounds pretty different from what I said, right? I have to admit that I’m not sure I can reconcile the two, though I will try.
Thing is, while I’m definitely not as qualified as the writers of Tobira, I’ve seen several Japanese sources (namely my dictionary, スーパー大辞林; this site: 「~なければいけない」と「~なければならない」; and this other one: 株式会社アルク) that don’t make the same distinction as Tobira. The two websites both say that ならない is used for duties, rules and things that are 当然 (obvious/natural), whereas いけない is used when the speaker considers that not fulfilling the obligation might be unpleasant or detrimental to somebody. One of them (ALC) notes that in Japanese laws, it’s more common to use ならない than いけない.
The dictionary, for its part, says ならない is for 当然の things (pardon my mixing of Japanese and English grammar) and duties, whereas いけない is used when ‘some reason or rule’ creates a ‘need or duty’ to do something. We can see that the dictionary isn’t exactly aligned with the websites, but notice that for いけない, a particular reason or constraint is needed as the source of the obligation, whereas for ならない, no particular justification is required. That isn’t too different from what the two Japanese sites mentioned.
All in all, I think it’s safe to say that the ‘external, natural obligation’ vs ‘personal, subjective obligation’ split is valid. How can we reconcile this with Tobira’s explanation, since the authors had no reason to put something false, particularly about practical usage, inside the book? I suspect (though I haven’t had enough exposure to Japanese conversation, so I can’t say for sure) that the reason it’s more common to use ならない to discuss one’s own obligations, while using いけない for those of others, is that it’s probably impolite or difficult to impose a universal/natural obligation on somebody else, whereas we can discuss our own obligations whichever way we want. Of course, what I’m saying is pure speculation, but I guess we’ll all find out when we start wading deeper into the world of Japanese speech. Either way, I have a feeling both Tobira and the other sources are right, it’s just that Tobira chose to give practical advice instead of a fundamental analysis of differences in meaning.