A friend of mine with 6yo twins read one called, I believe, "one and the same" that she loved. Then there's always Mark Twain's "Those Extraordinary Twins" if you want to be entertained. I believe that one's free as an e-book if you can find it.
One and the Same is an awesome book that explores the relationship between twins. However, its not really about raising twins.
I did find How to Raise Emotionally Healthy Twins somewhat helpful. The author definitely has an axe to grind but taken with a grain of salt, she encouraged me to look at issues of separateness, identity and self much earlier than I otherwise would. We are already seeing some emotional dependence between our 15 month old girls and are considering how we can give them some separate, independent time -- never would have even thought about the issue without having read that book.
We read a fantastic book called "Multiple Blessings" by Betty Rothbart. It starts with pregnancy and brings out some wonderful ideas. Even DH read it and liked it and he's not a reader at all. It was very informative and gave the pros and cons about being a twin, being an identicle, being a parent of a "unit", handling siblibngs of twins in the family, being a single sibling of twins, etc. We still find it a wonderful resource for anything that may crop up that we don't know how to deal with. Highly recommended.
Thank you for starting this thread! I was about to ask the same question. I've also heard "One and The Same" is a good read for understanding the relationship/psychology perspective.
Thank you! Now I'll start looking for the "One and the same" and probably also the "Multiple belssings".