One of the most common methods of cooking economically is OAMC, or once a month cooking. You buy huge amounts of groceries and cook up large quantities of several dishes, then freeze them in meal sized batches. This technique uses economical bulk buying and makes efficient use of the original meal preparation time. Maybe the biggest plus is that putting something in the oven to reheat is much easier at the end of a busy day than preparing something from scratch. If the technique sounds attractive, there are lots of recipes and guides online on how to do it well. The reason I don't use OAMC is that I can't get excited about reheated frozen meals on a regular basis, but if I had a job and a dozen kids I would probably resort to this method for at least a couple of nights a week.
The best book I have found on eating frugally is How to Cook a Wolf, by MFK Fisher. Rather than treating food as fuel and focusing on the biggest nutrition bang for buck, the book teaches how to be satisfied and generous in a way that is not high maintenance. Breakfast, for example, can be toast. You can use good grainy bread with lots of character and lash it with butter and honey, and it is still economical. Cooked oatmeal with raisins, brown sugar, milk, and half a banana usually keeps me going til late afternoon and still comes in under fifty cents per serving.
No matter what groceries cost, it is expensive to have things on hand that are not used. The cheapest way to shop is to not buy. The cheapest way to eat is to not eat. If you are not hungry and will have a chance to eat later, always eat later instead of now. Hunger is one of the biggest factors in whether eating is enjoyable or not, and the most overlooked.