Well, it's not such a wonderful book if you can't use it, is it? Your question is not really an issue of our measuring system being at odds with the metric system; it is that most recipes written for American cooks measure everything by volume, while recipes for British, Australian, South African, and other cooks measure most ingredients by weight. You said it yourself - "8 oz (225g) flour." That's not just metric, it starts out with ounces for heaven's sake - a measure we use in the United States. The problem is that you, like most of us, don't have a kitchen scale. We find a good scale to be an indispensable tool in the kitchen.
Even in this country, most professional bakers measure ingredients by weight. It is a much more accurate way to measure dry ingredients that can be loosely packed or tightly packed, and allows bakers to turn out consistent products day after day. A cup of flour can weigh anywhere between 3.5 ounces and 6 ounces, depending on the type of flour, how it has been packed, and how much humidity it has absorbed. Your recipe may be able to accommodate some variation, but if you've ever wondered why a dish turned out great one time and not so great another time, variation in your measurements could be a leading culprit.
Another complexity is that we do use the term ounces to measure liquids by volume rather than weight sometimes. If your recipe calls for 8 ounces of melted chocolate, you have been trained by the markings on your trusty glass measuring cup to assume that means 1 cup of melted chocolate. But 1 cup of melted chocolate weighs 10 ounces - so your 8 ounces actually weighs 10 ounces. How in blazes are you supposed to get it right? (A cup of water does weigh 8 ounces, which is why the measuring cup has ounce markings up the side. But all liquids do not weigh the same; honey and molasses weigh 12 ounces per cup.)
If you have a recipe in which ingredients are measured by weight rather than by volume, you have two practical choices: 1) ditch the recipe, or 2) get a scale and join the rest of the world. Otherwise, you would have to have an endless conversion chart for the weight of every given volume of every ingredient on earth. We do not have the patience to create such a chart and are not sure the Internet could hold it.
Most any scale on the market in this country registers both US and metric weights. If you come upon one that does not (you should think twice before buying it), to convert grams into ounces, multiply by 0.03527 and to convert ounces into grams, multiply by 28.35.
[Ed. note: The writer of this question found our answer to be glib, sarcastic, and mean spirited - apparently because we insulted her book, assumed that she does not own a kitchen scale, and/or made that get-with-the-rest-of-the-world comment. This was not our intention and we apologize for any harm we caused. Since she and every single friend she has is never going to visit Ochef ever, ever, ever, ever again, she may not actually see our apology, and for this we are also truly sorry. We may stand ourselves in the corner until she forgives us....]