Seriously, Positive Discipline by Jane Nelsen, it was a good find on a number of fronts.
This one?
Positive Discipline: The Classic Guide to Helping Children Develop Self-Discipline, Responsibility, Cooperation, and Problem-Solving Skills - Ebook written by Jane Nelsen, Ed.D.. Read this book using Google Play Books app on your PC, android, iOS devices. Download for offline reading, highlight...
play.google.com
That's the one. The main thing we took out of it I mentioned earlier in this thread - let your kid figure stuff out themselves as often as possible. Nelsen really emphasizes this point.
The 'positive' discipline aspect really boils down to positive responses being more effective than negative ones. Negative responses at best do nothing, at worst make the kid's behavior consistently worse, for a number of reasons. My wife and I are pretty laissez-faire in general, but my take is that the ultimate goal is to maintain your kid's respect and goodwill, so when they're older and you get to the serious stuff you actually have their attention. I think a lot of parents get caught up in the small stuff, that usually doesn't actually matter that much, and is generally just an artifact of their age.
The other thing the book does really well is describe sibling dynamics, and how position in the sibling hierarchy can play out. This has definitely informed how we treat our youngest. Nelsen claims that the youngest usually goes one of two ways, either they become a 'speeder' (always competing with older siblings and trying to catch up), or a 'baby' (expecting older siblings and parents to do everything). With your twins, I'm less sure how that'd play out, but I imagine they'd be equal in the hierarchy.
But I would recommend reading the book as it gets into nuance that will definitely help you understand child psychology.