I think that the most interesting aspect of having twins is it makes you realize there are many things you have no control over. So much of their personalities and traits are hard-wired. Unless you have two kids at the exact same time, you can't see how each has developed into their own person without any help from you.
You can't wonder what you did right with the first child that you must have forgotten to do with the second. You can't pat yourself on the back for fixing your mistakes with your second that you made with the first (even though you can't really recall what you did wrong). You actually have proof that you treated each child exactly the same, fed them the same food, put them to sleep using an identical method, etc. Even though all this is the same, at the end of the day they are completely different people. I have recently discovered, for example, that I have no control over the development of picky eating (Darn!).
So, I say throw out the books! Forget the professional advice! Just love and kiss them and see who they become.