Help with Book Navigation Changing

Hi all,

I'm working with content of two different types. I would like to add an additional item to the book navigation bar that:

1) Would only appear if user is reading something the next link points to Type A and;
2) Would allow the user to move two nodes ahead (that is, past the next node to the node immediately after).

It seems that I have to duplicate and modify the book_next function to begin with, but I'm having trouble making sense of how this function works. Any help would be most appreciated!