S.E.
I always recommend the book SIblings Without Rivalry. I know no one wants another book, but my girls are now 6 and 8 and I still refer back to it for pointers and mostly reminders to myself of what NOT to do. It is mostly in cartoon format so super easy to read and grasp. I constantly found myself saying - well geez that's obvious! But when the kids fought the exact opposite, or what my mother would have said, would STILL be what I would say..
Some of it is very hard to do esp now that they are older and come home with issues and I want SO to tell them how to fix it. I truly recommend this one.