I would view the book as an ABSOLUTE LAST RESORT when no other information is available. This is because, as Ginaki noted, it is a general, average value for similar products, AND because it is a point value (which is imprecise) for a specific serving size. It doesn't give you the information you need to figure out points for the amount YOU are having. For example, the book says 6 anchovy fillets are a point--which they probably are. But it turns out that a WHOLE CAN (drained) comes out to 2 points. If I counted out those fillets, I'd probably charge myself substantially more using the 6 fillets = 1 point. But something can be 1 point if it has 30 calories or 1 point if it has 70 calories. I use standard calorie reference books, websites (here's a great one:
www.calorieking.com), nutrition labels, etc.