Steph I would really recommend the book. It has a lot of great info in there. The authors are Dr. Barbara Luke and Tamara Eberlein. It does have a schedule of when to put the weight on, since moms of multiples don't get the full 40 weeks. Weight gain is dependent on your BMI, and it has an entire chart on that. Basically if you are less than 20 (I was a BMI of 18 to start with), then you need to gain 50-66 pounds! 20-24.9 needs to gain 40-56, BMI of 25-29.9 needs to gain 38-47, and above that is 31-36. Almost all of it needs to be put on before 28 weeks, because the amount you gain BEFORE then, affects the growth of the babies AFTER 28 weeks. Or so the book says
My friend had her twins at 31.5 weeks after gaining only 25 pounds, but her daughter's placenta was also only working at 10% capacity, so there were other circumstances involved. She did not follow the book, but says that most twin moms she knows who DID ended up with full size babies and no NICU time. Obviously there are other things that can lead to preemies (size of uterus, cervix, etc), but diet is one area we can control.
It also has a great chart of how much, and what, to eat (protein, carbs, etc), with a bunch of recipes.
So yea, if you fall in category #2 with the BMI, then it's saying you need to have put on 20-30 pounds by this point! CRAZY!! I wouldn't worry though... Just do everything you can to make it up. It also says to eat every 2-3 hours, so don't wait to get hungry. And make every bite count (though I do like my cookies, so...).
I am almost 5'8" and started at 118 pounds. I don't consider myself underweight because that weight fits my frame...I don't think every woman can take every chart the same way. However, my body has adapted really well to this pregnancy, and so far I'm putting on the weight just fine...though according to the book about 8 pounds or more behind, so I'm stepping it up!