Subtracting fiber from the total carbohydrates gives you your net carbs. The sugar content can be very low in some high carb foods, and very high in others.
I have a bag of chips here, they only have 1g of sugar per serving, but the net carbs are 17g. On a can of pineapple, the sugar is 15g, the net carbs are 15g.
As far as knocking you out of ketosis, it probably will for a day or two, but I personally don't use the strips. I did the first time I was on Atkins, but, I don't really think they are necessary. That's just me though.
I'm not sure what you are referring to as being different depending on the publishing date of the book. Net carbs have always been Total Carbs minus Dietary Fiber and Sugar Alcohols. Some people DO stall using sugar alcohols, but I personally have never experienced that, simply because I don't really use sweetners or eat a lot of sugar free stuff because they kind of make me queasy, not sure why.
Hope this helps!