Yippee!!! Congrats, Ill bet your head is spinning super hard. I remember the shock and awe after I found out and I was only 8 weeks :) There are ton of books listed on this thread, and you should join it anyway so I can put all your info on the main page.
Here are the multiples books I have read and my opinions on them (keep in mind that I dont have twins yet :) ):
When you're expecting twins, triplets and quads:
IMO, there is a lot of great advice in this book, and it was the first one I read so Im a bit partial to it. However, I think her dietary recommendations are insane. I just cannot eat as much (or honestly, even half as much) as she recommends. But, other than that, I think its a great book. It seems to be highly regarded as the "must have" twin book.
Its the LLL book for multiples moms. I loved it. My only problem with it, is the same problem I have with all of them. The author assumes that you are a first time mom. I wish there was a twin book out there that was titled something like, "What to do when youve read ten million parenting and pregnancy books and now you have TWO babies at once."
IMO, silly, and totally useless to me. I got it at the twin sale, and I read it because I was trying to get my hands on all the information I could, but I find it to be pretty much the opposite of all the opinions I have about parenting and pregnancy. But, its funny.
This one's my favorite. I think its super informational, medical wise, with a natural twist on it. I like the way she writes a lot.
Its got some good info in it. Its a great book to put on the back of the toilet so that DH will read it. It's broken up into small segments that can be read while in the bathroom :)
Then,depending on where you live, there is usually a local twins club nearby. http://www.nomotc.org/
My local club has a super active facebook and I've learned a ton from that.
Oh, and there's the twin moms here who have already had their twins (or triplets). They are the most helpful.