I'd recommend against getting (buying) a cookbook, as you can find recipes and food ideas for free on the internet. It sounds like you don't mind eating more or less the same things every day (more power to you!), but want to pay more attention to diet and health? Other than eating a variety of food groups, it's hard to give specific advice through the internet. This does sound like an issue best solved by finding someone knowledgable about food and learning from them, as Alicorn points out. It's also a good way to familiarize yourself with the basics of cooking, which is something that recipes tend to assume you have already.