If it’s a large Mass, the deacons walk first, followed by the priests, the main celebrant being last of course. However, the principle deacon walks immediately in front of the main celebrant. If not walking, the proper place for the deacon is to the celebrant’s right (the deacon is his "right hand). The deacon should not be both at the celebrant’s right and carrying the Gospels. I’m not sure what the pontifical says (See Dn Ed’s post) other than to repeat that there should be 2 or 3 main deacons.
I think the reason why there seems to be conflicting answers is because it matters whether or not the Gospel book is being carried. If he has the book, he’s always in front and not to the right. If he does not have the book, then either position is proper. Perhaps because it’s not always practical to have the 2 side-by-side, without the book, he can be in either position.