I cannot answer all your questions but maybe this will help anyway:

1) I do not know the book.
2) If the plans you are talking about are for a three bent barn then yes, you can just add two additional bents to achive your desired width.
3) Again, I do not know the book/plans.
4) Somewhere between $3000-$6000 depending on the wood species you are using and the cost of labor in you area.
5) Sheathing and roof underlayment is probably okay. I would not use it for the frame.
6)Yes, you can. But I would at least have footing where the posts are going to be.